Intercalation

Intercalation is the insertion of a leap day, week or month into some calendar years to make the calendar follow the seasons or moon phases. s may require a combination of both adjustments. The solar does not have a whole number of days, but a  must have a whole number of days. The only way to reconcile the two is to vary the number of days in the calendar year.

In solar calendars, this is often done by adding to a of 365 days, an extra day (leap day or intercalary day): this makes a  of 366 days.

The, which was issued by the pharaoh , Euergetes of in , decreed a solar leap day system.

In the as well as in the  that improved it, intercalation is done by adding an extra day to February in each. In the Julian Calendar this was done every 4 years. In the Gregorian calendar years whose number is evenly divisible by 100 but not 400, were exempted in order to improve accuracy.

The solar does not have a whole number of s either, so a  must have a variable number of s in a year. This is usually 12 months, but sometimes a 13th month (an intercalary or embolismic month) is added to the year.

includes a specification for a 52-week year. Any year that has 53 Thursdays has 53 weeks; this extra week may be regarded as intercalary.

The determination of whether a year has intercalation may be calculated (, and  calendars), or determined by observation.