Festivus is first mentioned in "The Strike," the 10th episode of the final season of Seinfeld. The episode, along with several others from the comedy series, is available to stream online, courtesy of Crackle. Each month, Crackle switches things up and provides 10 different episodes from the show.
Here's their synopsis for "The Strike":
George complains about Tim Whatley's making a donation in his name to a local charity, while Kramer learns that a 12-year-long strike at the bagel shop where he once worked is over. Kramer then inspires Frank to revive Festivus, a holiday he created as an alternative to Christmas.
Coined by George Constanza's father Frank, played by Jerry Stiller, Festivus takes place on Dec. 23, and is an alternative to the winter holidays. And as expected from Seinfeld, its key components are pretty wacky. Instead of a Christmas tree, there's a Festivus pole, essentially an unglamorous metal rod. There's also an "airing of grievances" tradition, where those who celebrate gather around before a meal to complain about each other, and the "feats of strength" competition, where the "man of the house" gets to choose an opponent and go head to head with them.
Watch the Seinfeld Festivus episode online for free here. Below you can check out how to make your own Festivus pole.