Jerry Seinfeld himself acknowledges that Season 3's "The Alternate Side" was the most uncomfortable episode of Seinfeld.
Festivus is an alternative holiday to Christmas celebrated by the Costanza family within the Seinfeld Universe. It was first introduced in Season 9, Episode 10, and was invented by Frank Constanza.
The Seinfeld Festivus episode occurs in Season 9, Episode 10, titled “The Strike ... Extra household member slots can be added onto a plan for $7.99 a month, as well.