Season 9, Episode 10: "The Strike"

-What? -Nothing. It's a card from my dad.

-What is it? -Elaine.

Dear son, Happy Festivus.

-What is Festivus? -It's nothing. Stop it.

-When George was growing-- -No.

His father hated all the commercial religious aspects of Christmas.

-Yeah? -He made up his own holiday.

Oh, and another piece of the puzzle falls into place.