According to Seinfeld, Festivus is celebrated each year on December 23, but many people celebrate it other times, often in early December. Its slogan is "A Festivus for the rest of us!"
This from Festivus web:
The Festivus celebration as described on Seinfeld includes four major components:
The Festivus Pole: The Costanzas' tradition begins with an aluminum pole, which Frank praises for its "very high strength-to-weight ratio." During Festivus, the unadorned Festivus Pole is displayed. The pole was chosen apparently in opposition to the commercialization of highly decorated Christmas trees, because it is "very low-maintenance," and also because the holiday's patron, Frank Costanza, "find[s] tinsel distracting."
The Airing of Grievances: At the beginning of the Festivus dinner, each participant tells friends and family of all the instances where they disappointed him or her that year. As quoted from Frank Costanza: "I've got a lot of problems with you people, and now you're going to hear about it!"
Festivus dinner: In "The Strike," a celebratory dinner is shown on the evening of Festivus prior to the Feats of Strength. The on-air meal appeared to be meat loaf or spaghetti in a red sauce. In Festivus: The Holiday for the Rest of Us by Allen Salkin, drinking is encouraged with hearty beer, rum, bourbon, or wine. In the episode, no alcohol was served, but George Costanza's boss, Mr. Kruger, drank from a flask.
The Feats of Strength: After the dinner, the head of the family tests his or her strength against one participant of the head's choosing. Festivus is not considered over until the head of the family has been pinned to the ground. A participant is allowed to decline to attempt to pin the head of the family only if they have something better to do instead.
Additionally: Exchanging gifts with your co-workers - When George Constanza was given a gift, he in turn gave his co-worker a card stating that a donation had been made in their name to a charity called The Human Fund and in fact this organization didn't exist...
The Wagner Companies, a Milwaukee-based maker of hand-railing components, is still producing its line of Festivus poles for the holiday season. Wagner offers a 6-foot Festivus pole for $39 and a 2-foot-8-inch tabletop model for $31. The setup is simple: a hollow pipe, 1.9 inches in diameter, inserted into a collapsible aluminum base.
There's a Festivus book website