February 31, with regard to the modern Western (revised Gregorian) calendar, is an imaginary date. It is sometimes used for example purposes, to make it clear regardless of context that the information being presented is artificial and not real data. February 30 is sometimes used in the same manner, although there are other calendars that legitimately use February 30.
HOUNSLOW, Sarah, his wife, d. 31.2.1835; six children, died in infancy;
Ann, wife of John Hounslow, d. 6.11.1890 [First wife's death date is impossible being 31st February which has only 28 days, or 29 in a Leap Year.]
"[T]he world's first February 31 Party! ... When: On February 31, of course! The festivities will start at 12:61 PM..." (usage as nonsense data in a domain name placeholder page).
An episode of The Alfred Hitchcock Hour based on the novel of the same name by Julian Symons is entitled "The Thirty-First of February".David Wayne stars as a man who, under suspicion of murdering his wife, is intentionally being driven insane to make him confess. The process includes changing his desk calendar back repeatedly to February 4, the date of her death, and eventually changing it to the nonexistent February 31.
"sample web page / Page Title / Last Updated February 31, 1999" (usage as example data in instructional materials).