What is Festivus?
Festivus is a nondenominational winter holiday which, in its popular form, originated on "The Strike," a 1997 episode of Seinfeld.
Festivus was described as a holiday invented by George's father, Frank Costanza, who had become disgusted with the commercialization of Christmas. Taking place on December 23rd, Festivus was celebrated with an unadorned aluminum pole instead of a tree. The family meal featured the "airing of grievances," in which Frank told the rest of the family how much they had disappointed him in the past year, and ended with the "feats of strength," which usually reduced George to tears.
The holiday is now celebrated by a number of Seinfeld fans, either out of devotion to the show, or out of a sense of absurdity, or because it provides an alternative to more established winter holidays that is utterly devoid of religious or political significance. Some celebrate it on Dec. 23; others on more convenient dates during the season.
The writers of Seinfeld loosely based the holiday on the Festivus celebrations created by the father of Dan O'Keefe, a writer on the show. This original Festivus occurred sporadically throughout the year, and did include the airing of grievances. It otherwise bears little resemblance to the Costanzas' version.
—The Fact Monster