It was a family tradition of Seinfeld writer Dan O’Keefe, whose father created Festivus as early as 1966.
Festivus has since been adopted as a pseudo-holiday by real people, spurning a number of books and think pieces about the influences of Festivus.
The real version has some differences from the TV one, mainly that instead of an aluminum pole, the O’Keefe family put a clock in a sack and nailed it to the wall.
The funny thing about the holiday, though, is that it was actually real for many years before winding up in a Seinfeld episode.