Festivus is a holiday celebrated on December 23rd, created by Frank Costanza as an alternative to Christmas. He came upon the idea after pummeling a man in the face in a toy-store dispute over a doll (for George). In his own words, "As I rained blows upon him, I realized, 'There had to be another way!'"
It features an aluminum pole instead of a Christmas tree due to Frank finding tinsel "distracting." At dinner, the Airing of Grievances is conducted, in which the celebrants tell each other what the other has done to irritate them that year. The celebration of Festivus does not end until the Feats of Strength are performed, usually involving George being forced to pin his father.
After Kramer became interested in the idea of "A Festivus for the rest of us," he inspired Frank to begin celebrating the holiday once more, causing great anguish for George, who loathed Festivus (the Feats of Strength, in particular). Kramer, Jerry, Elaine, George, Mr. Kruger (George's boss, who had become very interested in Festivus after hearing it mentioned), and several other guests appeared, causing a very socially awkward situation.
A blog dedicated to the legality of the issues that arise in Seinfeld episodes, Seinfeld Law , analyzed whether Kramer was legally entitled, under Title VII of the Civil Rights Act, to take time off for Festivus.