Unlock Frankenbart:
To unlock a Halloween version of Bart Simpson, set your PS2's date to 10/31. Bart will appear as Frankenstein on the character select screen (under the ? category).

Unlock Holiday Krusty:
To unlock a special holiday outfit for Krusty, set your PS2's clock to January 1. Krusty, in his spiffy duds, will be selectable from the Character Selection screen.

Unlock Marge the Pilgrim:
To see Marge all gussied up like a Pilgrim, set your PS2's date to Thanksgiving day (November 22). Marge will be selectable at the character select screen.

Unlock Santa Apu:
To see Apu dressed as Santa Claus, set your PS2's date to December 25. The jolly Kwik-E Mart owner will then be available on the character select screen.