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.
In this respect, these "dates" are similar to other clearly fictional data used for a similar purpose, such as the 12th of Never or "John Q. Public".
Examples of deliberate usage 
This gravestone in the Old Mission Church Cemetery in Upper Sandusky, Ohio
, shows the date of death for Christiana Haag as February 31, 1869.
HOUNSLOW, Sarah, his wife, d. 31.2.1835; six children, died in infancy;
Ann, wife of John Hounslow, d. 6.11.1890
- "[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).
Ripley's Believe It or Not! states that William O'Malley, "according to his birth certificate on file in Clifden, Ireland, was born in 1853 on February 31".
See also