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The Latin word “festus”, means “joyous; holiday, feast day”.
The date of Festivus was chosen to celebrate the anniversary of Daniel O’Keefe’s first date with his future wife, Deborah.

In 1982, Daniel O’Keefe mentioned Festivus in his book “Stolen Lightning: The Social Theory of Magic”, dealing with idiosyncratic rituals and its social significance, which is a theme relevant to Festivus tradition.

The holiday became extra popular in 1997 after being mentioned in the Seinfeld episode “The Strike” which was co-written by  O’Keefe’s son, Dan O’Keefe.

Festivus is celebrated by erecting a festivus pole, which is simply an aluminium pole without decorations.
A dinner is has as with most winter holidays.
After the dinner a traditional “Airing of Grievances” is done, followed by “Feats of Strength” which is often a wrestling match resulting from the former ritual.
Other more creative challenges may also be done.

Sources:

• https://en.wikipedia.org/wiki/Festivus