From the NYT via No More Mr. Nice Blog:
The first surprise is that from Tampa Bay, Fla., to Washington, from
Austin, Tex., to Oxford, Ohio, many real people are holding parties celebrating Festivus, a holiday most believe was invented on an episode of "Seinfeld" first broadcast the week before Christmas in 1997.
...
The actual inventor of Festivus is Dan O'Keefe, 76, whose son Daniel, a writer on "Seinfeld," appropriated a family tradition for the episode. The elder Mr. O'Keefe was stunned to hear that the holiday, which he minted in 1966, is catching on. "Have we accidentally invented a cult?" he wondered.
...
"It was entirely more peculiar than on the show," the younger Mr. O'Keefe said from the set of the sitcom "Listen Up," where he is now a writer. There was never a pole, but there were airings of grievances into a tape recorder and wrestling matches between Daniel and his two brothers, among other rites."There was a clock in a bag," said Mr. O'Keefe, 36, adding that he does not know what it symbolized.