Oh, I remember applesauce day now! Took a little nudge, though. D;
My favorite by far was the '09 Todoroki. I remember that thread in OT that would ask you to guess the Todoroki above. I'd cheat by logging out and checking, cause everything returned to normal if you did. :3
Personally, I think a user-switch around day would be pretty awesome, though that seems improbable. Like, I'm usually Bloodex, but for April 1st, I'd be the username of the person who registered next on the database for the day. Make sense?