Gregorian calendar

The Gregorian calendar is the most widely used in the world today. A modification of the, it was first proposed by the n doctor , and was decreed by , for whom it was named, on  via the  . Years in the calendar continue the numbering of the Julian calendar, which are numbered from the traditional birth year of, which has been labeled the "" (AD) era, and is sometimes labeled the "" or the "Christian Era" (CE).

The changes made by the Gregorian calendar was to correct the drift in the because the mean  year was slightly too long, causing the, and consequently the , to slowly drift forward in relation to the civil calendar.

The Gregorian calendar system dealt with these problems by dropping 10 days to bring the calendar back into synchronization with the seasons, and adopting the following leap year rule: "Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years. For example, the year 1900 is not a leap year; the year 2000 is a leap year." In the Julian calendar, all years exactly divisible by 4 were leap years.

Description
The Gregorian solar calendar is an arithmetical calendar. It counts days as the basic unit of time, grouping them into years of 365 or 366 days. The solar calendar repeats completely every 146,097 days, which fill 400 years, and which also happens to be 20,871 seven-day s. Of these 400 years, 303 (the "common years") have 365 days, and 97 - the s - have 366 days. This gives an average year length of exactly 365.2425 days - or 365 days, 5 hours, 49 minutes and 12 seconds.

A Gregorian year is divided into twelve s of irregular length (but note that there is a period of 153 days divided over 5 months in an alternating pattern from March to July that repeats from August to December):

A calendar date is fully specified by the year (numbered by some scheme beyond the scope of the calendar itself), the month (identified by name or number), and the day of the month (numbered sequentially starting at 1).

s are all years divisible by 4, with the exception of those divisible by 100, but not by 400. These 366-day years add a 29th day to February, which normally has 28 days. Thus, the essential ongoing differential feature of the Gregorian calendar, as opposed to the Julian calendar, is that the Gregorian omits 3 leap days every 400 years. This difference would have been more noticeable in modern memory were it not for the fact that the year 2000 was a leap year in both the Julian and Gregorian calendar systems.

The day in a leap year is known as a. Since Roman times  was counted as the leap day, but nowadays  is regarded as the leap day in most countries.

Although the calendar year runs from to, sometimes year numbers were based on a different starting point within the calendar. Confusingly, the term "Anno Domini" is not specific on this point, and actually refers to a family of year numbering systems with different starting points for the years. (See the section below for more on this issue.)

Gregorian reform
The motivation of the in adjusting the calendar was to celebrate  at the time it thought the  had agreed upon in. Although a canon of the council implies that all churches used the same Easter, they did not. The Church of Alexandria celebrated Easter on the Sunday after the 14th day of the Moon (computed using the ) that falls on or after the, which they placed on. However, the Church of Rome still regarded as the equinox and used a different cycle to compute the day of the Moon. By the tenth century all churches (except for some on the eastern border of the ) had adopted the Alexandrian Easter, which still placed the vernal equinox on, although had already noted its drift in 725—it had drifted even further by the sixteenth century.

Worse, the reckoned Moon that was used to was fixed to the Julian year by a. However, that approximation built up an error of one day every 310 years, so by the sixteenth century the lunar calendar was out of phase with the real Moon by four days.

The approved a plan in  for correcting the calendrical errors, requiring that the date of the  be restored to that which it held at the time of the First Council of Nicaea in 325 and that an alteration to the calendar be designed to prevent future drift. This would allow for a more consistent and accurate scheduling of the feast of.

The fix was to come in two stages. First, it was necessary to approximate the correct length of a solar year. The value chosen was 365.2425 days in decimal notation. This is 365;14,33 days in notation—the length of the, rounded to two sexagesimal positions; this was the value used in the major astronomical tables of the day. Although close to the of 365.24219 days, it is even closer to the  of 365.2424 days; this fact made the choice of approximation particularly appropriate as the purpose of creating the calendar was to ensure that the vernal equinox would be near a specific date. (See ).

The second stage was to devise a model based on the approximation which would provide an accurate yet simple, rule-based calendar. The formula designed by was ultimately successful. It proposed a 10-day correction to revert the drift since Nicaea, and the imposition of a leap day in only 97 years in 400 rather than in 1 year in 4. To implement the model, it was provided that years divisible by 100 would be s only if they were divisible by 400 as well. So, in the last millennium, 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not. In this millennium, 2100, 2200, 2300 and 2500 will not be leap years, but 2400 will be. This theory was expanded upon by in a closely argued, 800 page volume. He would later defend his and Lilius's work against detractors.

The 19-year cycle used for the lunar calendar was also to be corrected by one day every 300 or 400 years (8 times in 2500 years) along with corrections for the years (1700, 1800, 1900, 2100 et cetera) that are no longer leap years. In fact, a new method for was introduced.

Lilius originally proposed that the 10-day correction should be implemented by deleting the Julian leap day on each of its ten occurrences during a period of 40 years, thereby providing for a gradual return of the equinox to. However, Clavius's opinion was that the correction should take place in one move and it was this advice which prevailed with Gregory. Accordingly, when the new calendar was put in use, the error accumulated in the 13 centuries since the Council of Nicaea was corrected by a deletion of ten days. The last day of the Julian calendar was Thursday  and this was followed by the first day of the Gregorian calendar, Friday   (the cycle of weekdays was not affected).

Adoption


Though Gregory's reform was "enacted" in the most solemn of forms available to the Church, in fact the had no authority of its own. The changes which he was "proposing" were changes to the over which he had no authority. The changes required "adoption" by the civil authorities in each country to have legal effect.

Only four (Catholic) countries adopted the new calendar on the date specified by the bull. Other Catholic countries experienced some delay before adopting the reform; and non-Catholic countries, not being subject to the decrees of the Pope, initially rejected or simply ignored the reform altogether, although they all eventually adopted it. Hence, the dates " " to " " (inclusive) are still valid dates in many countries.

,, the , and most of implemented the new calendar on Friday,  , following Julian Thursday,. The Spanish and Portuguese colonies adopted the calendar later due to the slowness of communication in those days. adopted the new calendar on Monday,, following Sunday,. The Protestant provinces of Holland and Zeeland also adopted it in December of that year.

Most non-Catholic countries initially objected to adopting a Catholic invention, especially during the (of which Gregory was a leading proponent); some  feared the new calendar was part of a plot to return them to the Catholic fold. In the Czech lands, Protestants resisted the calendar imposed by the. In parts of, Catholic rebels till their defeat in the kept the "new" Easter in defiance of ; later Catholics practising in secret petitioned the  for  from observing the new calendar, as it signalled their disloyalty.

, and the Protestant states of  adopted the solar portion of the new calendar on Monday, , following Sunday,  , due to the influence of , but did not adopt the lunar portion. Instead, they decided to calculate the date of Easter astronomically using the instant of the vernal equinox and the full moon according to 's Rudolphine Tables of 1627. They finally adopted the lunar portion of the Gregorian calendar in 1776. The remaining provinces of the also adopted the Gregorian calendar in 1700.

's relationship with the Gregorian Calendar had a difficult birth. Sweden started to make the change from the OS calendar and towards the NS calendar in 1700, but it was decided to make the (then 11 day) adjustment gradually, by excluding the leap days from each of 11 successive leap years, 1700 to 1740. In the meantime, not only would the Swedish calendar be out of step with both the Julian calendar and the Gregorian calendar for 40 years, but also the difference would not be static but would change every 4 years. This strange system clearly had great potential for endless confusion when working out the dates of Swedish events in this 40 year period. To make matters worse, the system was poorly administered and the leap days that should have been excluded from 1704 and 1708 were not excluded. The Swedish calendar should by now have been 8 days behind the Gregorian, but it was still in fact 10 days behind. King wisely recognised that the gradual change to the new system was not working and he abandoned it. However, rather than now proceeding directly to the Gregorian calendar (as in hindsight seems to have been the sensible and obvious thing to do), it was decided to revert to the Julian calendar. This was achieved by introducing the unique date in the year 1712, adjusting the discrepancy in the calendars from 10 back to 11 days. Sweden finally adopted the Gregorian calendar in 1753, when Wednesday, was followed by Thursday,. Since Finland was under Swedish rule at that time, it did the same.

and the (including the eastern part of what is now the ) adopted the Gregorian calendar in 1752 (see the )  by which time it was necessary to correct by 11 days. Wednesday,  was followed by Thursday,   to account for   (Julian). After 1753, the British tax year in Britain continued to operate on the Julian calendar and began on, which was the "old style" new tax year of. A 12th skipped Julian leap day in 1800 changed its start to. It was not changed when a 13th Julian leap day was skipped in 1900, so the tax year in the still begins on.

In, the change took place when Friday, , was followed again by Friday,  after the US purchase of Alaska from Russia, which was still on the Julian calendar. Instead of 12 days, only 11 were skipped, and the day of the week was repeated on successive days, because the was shifted from Alaska's eastern to western boundary along with the change to the Gregorian calendar.

In the Gregorian calendar was accepted after the  (so named because it took place in October 1917 in the Julian calendar). On  the  issued a  that Wednesday,   was to be followed by Thursday,.

The last country of Eastern Orthodox Europe to adopt the Gregorian calendar was on Thursday, , following Wednesday,. However, these were all civil adoptions—none of the national churches accepted it. Instead, a was proposed in May 1923 which dropped 13 days in 1923 and adopted a different leap year rule that resulted in no difference between the two calendars until 2800. The Orthodox churches of, , , , , , , and adopted the Revised Julian calendar, so these  would celebrate the  along with the Western churches on  in the Gregorian calendar until 2800. The Orthodox churches of, , , and the  did not accept the Revised Julian calendar. These continue to celebrate the Nativity on  in the Julian calendar, which is  in the Gregorian calendar until 2100. All of the other Eastern churches, the churches  and the, continue to use their own calendars, which usually result in fixed dates being celebrated in accordance with the Julian calendar. All Eastern churches continue to use the Julian Easter with the sole exception of the, which has adopted the Gregorian Easter.

Adoption in Eastern Asia
The (ROC) formally adopted the Gregorian calendar at its founding on , but China soon descended into a period of warlordism with different warlords using different calendars. With the of China under the  in October 1928, the  decreed that effective   the Gregorian calendar would be used thenceforth. However, China retained the Chinese traditions of numbering the months and a modified, backdating the first year of the ROC to 1912; this system is still in use in where this ROC government retains control. Upon its foundation in 1949, the continued to use the Gregorian calendar with numbered months, but abolished the ROC Era System and adopted the Western fashion of naming years.

replaced the traditional lunisolar calendar with the Gregorian calendar on, but, like China, continued to number the months, and used reign names instead of the : Meiji 1=1868, Taisho 1=1912, Showa 1=1926, Heisei 1=1989, and so on. The "Western calendar" (西暦, seireki) using western year numbers, is also widely accepted by civilians and to a lesser extent by government agencies.

started using the Gregorian calendar on  due to Japanese influence. The lunisolar used immediately before that day was based on the lunisolar.

Difference between Gregorian and Julian calendar dates
Since the introduction of the Gregorian calendar, the difference between faster Gregorian (New Style) and slower Julian (Old Style) calendar dates has increased by three days every four centuries:

Beginning of the year
The had begun their years on. During the under the influence of the Christian Church, many countries moved the start of the year to one of several important Christian festivals &mdash; (the ),, (the ), or even. Eastern European countries (most of them with populations showing allegiance to the ) began their numbered year on from about.

In England was celebrated as the New Year festival, but from the 12th century to 1752 the year in England began on. So for example the Parliamentary record records the execution of occurring in 1648, (as the year did not end until 24 March,) although modern histories adjust the start of the year to January 1 and record the execution as occurring in 1649.

Most Western European countries changed the start of the year to before they adopted the Gregorian calendar. For example Scotland changed the start of the Scottish New Year to in 1600 (this means that 1599 was a short year). England, Ireland and the British colonies changed the start of the year to in 1752, (so 1751 was a short year with only 282 days). Later that year in September the Gregorian calendar was introduced throughout Britain and the British colonies (See the section ). These two reforms were implemented by the.

Neither the papal bull nor its attached canons explicitly fix such a date, though it is implied by two tables of 's days, one labeled 1582 which ends on, and another for any full year that begins on. It also specifies its relative to, in contrast with the Julian calendar, which specified it relative to. These would have been the inevitable result of the above shift in the beginning of the Julian year.

During the period between 1582, when the first countries adopted the Gregorian calendar, and 1923, when the last European country adopted it, it was often necessary to indicate the date of some event in both the Julian calendar and in the Gregorian calendar, for example, "10/21 February 1751/52", where the dual year accounts for some countries already beginning their numbered year on while others were still using some other date. Even before 1582, the year sometimes had to be double dated because of the different beginnings of the year in various countries. Woolley, writing in his biography of (1527-1608/9), notes that immediately after 1582 English letter writers "customarily" used "two dates" on their letters, one OS and one NS.

Old Style and New Style dates
"Old Style" (OS) and "New Style" (NS) are sometimes added to dates to identify which system is used in the and other countries that did not immediately change. Because the Calendar Act of 1750 altered the start of the year, and also aligned the British calendar with the Gregorian calendar, there is some confusion as to what these terms mean. They can indicate that the start of the has been adjusted to start on  (NS) even though contemporary documents use a different start of year (OS); or to indicate that a date conforms to the Julian calendar (OS), formerly in use in many countries, rather than the Gregorian calendar (NS). More details and examples are available in the  article.

Proleptic Gregorian calendar
The Gregorian calendar can, for certain purposes, be extended backwards to dates preceding its official introduction, producing the. However, this proleptic calendar should be used with great caution.

For ordinary purposes, the dates of events occurring prior to  are generally shown as they appeared in the Julian calendar, with the year starting on, and no conversion to their Gregorian equivalents. The is universally known to have been fought on   which is 's Day.

Usually, the mapping of new dates onto old dates with a start of year adjustment works well with little confusion for events which happened before the introduction of the Gregorian Calendar. But for the period between the first introduction of the Gregorian calendar on  and its introduction in Britain on , there can be considerable confusion between events in continental western Europe and in British domains in English language histories. Events in continental western Europe are usually reported in English language histories as happening under the Gregorian calendar. For example the is always given as. However confusion occurs when an event affects both. For example arrived at  in England on  (Julian calendar), after setting sail from the  on  (Gregorian calendar).

and apparently died on exactly the same date, but in fact Cervantes predeceased Shakespeare by ten days in real time (for dating these events, Spain used the Gregorian calendar, but Britain used the Julian calendar). This coincidence however has allowed to make  the.

Astronomers avoid this ambiguity by the use of the.

For dates before the year 1, unlike the proleptic Gregorian calendar used in the, the traditional proleptic Gregorian calendar (like the Julian calendar) does not have a  and instead uses the ordinal numbers 1, 2, … both for years AD and BC. Thus the traditional timeline is 2 BC, 1 BC, AD 1, and AD 2. ISO 8601 uses which includes a year 0 and negative numbers before it. Thus the ISO 8601 timeline is -0001, 0000, 0001, and 0002.

Months of the year
English speakers sometimes remember the number of days in each month by the use of the traditional verse:


 * Thirty days hath September,
 * April, June, and November.
 * All the rest have thirty-one,
 * excepting February alone,
 * which hath twenty-eight.
 * Leap year cometh one year in four,
 * in which February hath one day more.

(The hath in the first line of the poem is also given as has or have.)

Alternate endings include:


 * except for February alone,
 * which has twenty-eight days each year,
 * and twenty-nine days each leap year.


 * excepting February alone,
 * which has twenty-eight days or,
 * in a leap year, adds one more.


 * which has but twenty-eight, in fine,
 * till leap year gives it twenty-nine.


 * which has eight and a score,
 * until leap year gives it one day more.


 * which hath twenty-eight days clear,
 * and twenty-nine in each leap year.


 * in each leap we assign,
 * February twenty-nine.


 * When short February's done,
 * all the rest have thirty-one.


 * (except February,)
 * February alone don't hold the line,
 * for three years it has twenty-eight,
 * and the fourth year twenty-nine.


 * but February, it is done
 * at twenty-eight, but add one more
 * whenever the year divides by four.

A shorter, satirical modern alternate ending is:
 * but silly old February spoils the fun.

A language-independent alternative used in many countries is to hold up your two fists with the index knuckle of your left hand against the index knuckle of your right hand. Then, starting with January from the little knuckle of your left hand, count knuckle, space, knuckle, space through the months. A knuckle represents a month of 31 days, and a space represents a short month (a 28- or 29-day February or any 30-day month). The junction between the hands is not counted, so the two index knuckles represent July and August. This method also works by starting the sequence on the right hand's little knuckle, and continue toward to the left. You can also use just one hand; after counting the fourth knuckle as July, start again counting the first knuckle as August. A similar mnemonic can be found on a : starting on the key F for January, moving up the keyboard in s, the black notes give the short months, the white notes the long ones.

The Origins of English naming used by the Gregorian calendar:
 * January: (Roman god of gates, doorways, beginnings and endings)
 * February: ( god of death)  Februarius (mensis) (Latin for "month of purification (rituals)" it is said to be a Sabine word, the last month of ancient pre-450 BC ). It is related to.
 * March: (Roman god of war)
 * April: Aprilis (mensis) (Latin for "month of ," second month of ancient Roman calendar)
 * May: (Roman goddess)
 * June: (Roman goddess, wife of Jupiter)
 * July: (Roman dictator) (month was formerly named Quintilis, the fifth month of the calendar of )
 * August: (first Roman emperor) (month was formerly named Sextilis, the sixth month of Romulus)
 * September: septem (Latin for seven, the seventh month of Romulus)
 * October: octo (Latin for eight, the eighth month of Romulus)
 * November: novem (Latin for nine, the ninth month of Romulus)
 * December: decem (Latin for ten, the tenth month of Romulus)

Week
In conjunction with the system of months there is a system of s. A physical or electronic calendar provides conversion from a given date to the, and shows multiple dates for a given weekday and month. is not very simple, because of the irregularities in the Gregorian system. When the Gregorian calendar was introduced, the week cycle was continued unbroken. So Thursday,  was followed by Friday.

The connects Gregorian years and weeks, defining a  with so-called "ISO years" deviating at the beginning and end up to 3 days from Gregorian years, and with week numbers by year.

Origins of English week day names used by the Gregorian Calendar:


 * Monday - day (celestial), a modernization of "Monnendaeg"
 * Tuesday - 's day (Old Norse god - Tiw in Old English, Teiw in Proto-Germanic)
 * Wednesday - 's day (Old English god - Norse, German )
 * Thursday - 's day (Old Norse god)
 * Friday - 's day (Old Norse goddess) (Friday is often erroneously associated with )
 * Saturday - 's day (Roman god)
 * Sunday - day (celestial), a modernization of "Sunnendaeg"

Distribution of dates by day of the week
Since the 400-year cycle of the Gregorian calendar consists of a whole number of weeks, each cycle has a fixed distribution of weekdays among calendar dates. It then becomes possible that this distribution is not even.

Indeed, because there are 97 leap years in every 400 years in the Gregorian Calendar, there are on average 13$6/7$ for each starting weekday in each cycle. This already shows that the frequency is not the same for each weekday, which is due to the effects of the "common" centennial years (1700, 1800, 1900, 2100, 2200 etc.).

The absence of an extra day in such years causes the following leap year (1704, 1804, 1904, 2104 etc.) to start on the same day of the week as the leap year twelve years before (1692, 1792, 1892, 2092 etc.). Similarly, the leap year eight years after a "common" centennial year (1708, 1808, 1908, 2108 etc.) starts on the same day of the week as the leap year immediately prior to the "common" centennial year (1696, 1796, 1896, 2096 etc.). Thus, those days of the week on which such leap years begin gain an extra year or two in each cycle. In each cycle there are:


 * 13
 * 14
 * 14
 * 13
 * 15
 * 13
 * 15

Note that as a cycle, this pattern is symmetric with respect to the low Saturday value.

A leap year starting on Sunday means the next year does not start on Monday, so more leap years starting on Sunday means fewer years starting on Monday, etc. Thus the pattern of number of years starting on each day is inverted and shifted by one weekday: 56, 58, 57, 57, 58, 56, 58 (symmetric with respect to the high Sunday value).

The number of common years starting on each day is found by subtraction: 43, 44, 43, 44, 43, 43, 43.

The frequency of a particular date being on a particular weekday can easily be derived from the above (for dates in March and later, relate them to the next New Year).

See also.

Accuracy
The Gregorian calendar improves the approximation made by the by skipping three Julian leap days in every 400 years, giving an average year of 365.2425 s long, which has an error of about one  per 3300 s with respect to the  of 365.24219 days but less than half this error with respect to the  of 365.24237 days. Both are substantially more accurate than the one day in 128 years error of the Julian calendar (average year 365.25 days).

In the 19th century, Sir proposed a modification to the Gregorian calendar with 969 leap days per 4000 years, instead of 970 leap days that the Gregorian calendar would insert over the same period. This would reduce the average year to 365.24225 days. Herschel's proposal would make the year 4000 common instead of leap. While this modification has often been proposed since, it has never been officially adopted.

On timescales of thousands of years, the Gregorian calendar falls behind the seasons drastically because the slowing down of the Earth's rotation makes each day slightly longer over time (see and ) while the year maintains a more uniform duration. It was calculated that the equinox will occur earlier than now by a number of days approximately equal to $$\left(\frac{\text{years into future}}{5000}\right)^{2}$$. This is a problem that the Gregorian calendar shares with any rule-based calendar with a fixed cycle. (However, recent evidence suggests that melting of glaciers (resulting from global warming) may create sufficient movement of water from high altitudes to the oceans to reverse the slowing, to satisfy the law of conservation of angular momentum. )

Calendar seasonal error


This image shows the difference between the Gregorian calendar and the seasons.

The y-axis is "days error" and the x-axis is Gregorian calendar years.

Each point represents a single date on a given year. The error shifts by about a quarter of a day per year. Years that are multiples of 100 but not 400 are not leap years. This causes a correction on years 1700, 1800, 1900, 2100, 2200, and 2300.

For instance, these corrections cause  to be the latest December solstice, and   to be the earliest solstice&mdash;2.25 days of variation compared with the seasonal event.

Numerical facts
When leap years and common years are taken into account, there are a total of 14 possible Gregorian calendars.

When different dates of Easter are also taken into account, there are a total of 70 possible Gregorian calendars.

An average year is 365.2425 days = 52.1775 weeks = 8,765.82 hours = 525,949.2 minutes = 31,556,952 seconds. All these numbers are exact, apart from leap seconds.

A common year is 365 days = 8,760 hours = 525,600 minutes = 31,536,000 seconds.

A leap year is 366 days = 8,784 hours = 527,040 minutes = 31,622,400 seconds.

Since 1971, some years may also contain one or more s, to account for cumulative irregularities in the Earth's rotation. So far, these have always been positive and have occurred on average once every 18 months.

The day of the year is somewhat inconvenient to compute, not in the least because of the leap day somewhere in the middle; but the calendar has this repeating pattern for the months March through July and August through December: 31, 30, 31, 30, 31 days, totalling 153 days. In fact, any 5 consecutive months not containing February, count 153 days. happens to be the 17th, and the sum of the first 5 s (among other numerical trivia).

See also and.

The 400-year cycle of the Gregorian calendar has 146,097 days and hence exactly 20,871 weeks. So, for example, the days of the week in Gregorian 1603 were exactly the same as for 2003. The years that are divisible by 400 begin on a Saturday. In the 400-year cycle, more months begin on a Sunday (and hence have ) than any other day of the week (see above under for a more detailed explanation of how this happens). 688 out of every 4800 months (or 172/1200) begin on a Sunday, while only 684 out of every 4800 months (171/1200) begin on each of Saturday and Monday, the least common cases.

A smaller cycle is 28 years (1,461 weeks), provided that there is no dropped leap year in between. Days of the week in years may also repeat after 6, 11, 12, 28 or 40 years. Intervals of 6 and 11 are only possible with common years, while intervals of 28 and 40 are only possible with leap years. An interval of 12 years only occurs with common years when there is a dropped leap year in between.

The is a method by which you can discern which of the 14 calendar variations should be used in any given year (after the Gregorian reformation). It is based on the last day in February, referred to as the Doomsday.

The Gregorian serial date, also called Rata Die, is the number of days from,  (counting that day as day 1). For August 28, 2024, the serial date is . It is 678576 more than the Modified, and 1721425 less than the Julian date.

Trivia
The was modified by  when he occupied the office of  and the  was subsequently modified by, who, as Pope, also held the title Pontifex Maximus.

Non-leap years always begin and end on the same day of the week, since 364 (365 - 1) is a multiple of 7, the number of days in a week. For example, 2003 began on a Wednesday and ended on a Wednesday. Leap years end on the next day of the week from which they begin. For example, 2004 began on a Thursday and ended on a Friday.

Not counting leap years, any calendar date will move to the next day of the week the following year. For example, if your birthday fell on a Tuesday in 2002, it fell on a Wednesday in 2003. Leap years make things a little more complicated. 2004 was a leap year, so calendar days of or later in the year, moved two days of the week from 2003. However, calendar days occurring before do not make the extra day of the week jump until the year following a leap year. So, if your birthday is, then it must have fallen on a Sunday in 2003 and a Tuesday in 2004. If, however, your birthday is, then it must have fallen on a Saturday in 2003, a Sunday in 2004 and a Tuesday in 2005.

In any year (even a leap year), July always begins on the same day of the week that April does. Therefore, the only difference between a July calendar page and an April calendar page in the same year is the extra day July has. The same relationship exists between September and December as well as between March and November. Add an extra day to the September page and you've got December. Take a day away from the March page and you've got November. In non-leap years only, there are additional matches: October duplicates January, and March and November duplicate February in their first 28 days. In leap years only, there is a different set of additional matches: July is a duplicate of January while February is duplicated in the first 29 days of August.

died on the night from to , that is, exactly when Spain and the Catholic world switched to the Gregorian calendar.

and died on exactly the same date,. However, since England had not adopted the Gregorian calendar by that time, they did not die the same day. In their honor, UNESCO declared it.

The athletes sent to the   in  arrived two weeks late, since their country was still using the Julian Calendar and nobody had advised them of the change.