Festivus is an annual holiday invented in ancient Rome and adapted to modern times by the writer and editor of Reader's Digest, Dan O'Keefe. It was introduced into popular culture by O'Keefe's son, a screenwriter of the series "Seinfeld" on December 18, 1997, in the episode "The Strike" (Season 9, Episode 10).
The holiday is celebrated every year on December 23, but many people celebrate it at other times, often to avoid the Christmas rush.
It includes such practices as the "Airing of Grievances" in which each person tells everyone else all the ways they have disappointed over the past year, and after a Festivus dinner, the "Feats of Strength" are performed, the head of household puts its power to prove against a person of your choice. Participants may refuse the challenge if and when they have something better to do.
Many people, influenced or inspired by Seinfeld, now celebrate the holiday, in varying degrees of severity. Some carefully following the rules of the series or books, others playfully invent their own versions.