Celebrated on December 23, the day before Christmas Eve, the holiday involves a metal pole (instead of a Christmas tree). Instead of exchanging gifts, participants are encouraged to air their grievances. After that comes the feats of strength (pushups, for example). The party continues until the head of the family is wrestled to the floor.
The Seinfeld cast explains Festivus above. Here are 9 1/2 fun facts about Festivus.