Unlock Frankenbart
To unlock Frankenbart (a Halloween version of Bart Simpson), set your GameCube's date to October 31. Bart will appear as Frankenstein on the character select screen (under the ? category).

Unlock Marge the Pilgrim
To unlock Marge dressed up like a Pilgrim, set your GameCube's date to Thanksgiving day (November 22). Marge will be selectable at the character select screen.

Unlock Santa Apu
To unlock Apu dressed as Santa Claus, set your GameCube's date to December 25. Santa Apu will then be available on the character select screen.

Unlock New Year's Krusty the Clown
To unlock a special new year's outfit for Krusty the Clown, set your GameCube's clock to January 1. New Year's Krusty will be selectable from the Character Selection screen.