Frank Costanza created the holiday "a Festivus for the rest of us" after getting into a fight with a fellow shopper over a doll for his son (no wonder George is so weird.
Festivus is celebrated in Dec 23 and higjlights include an aluminium pole instead of a Christmas tree (Frank finds tinsel "distracting") and a family meal which kicks off with the "airing of grievances" and ends in the "feats of strength".
The funniest bit of this episode for me is Frank's voice-over at the end after he's chosen to do battle with George in the feats of strength. "George, stop crying and fight your father." Priceless!