Tonight is definetly number one here in St. Louis.
I think it differs by region though. So my guess, in order

Night before thanksgiving (i know this is #1 here)

Mardi Gras (we have the number 2 mardi gras party in the nation, maybe number one, after Katrina)

St. Paddys day (possibly). The third one is a little tougher, I dont think its superbowl, or new years though.

And Ill agree with the sentiment that the amateurs come out on new year, tonight is for the real drinkers...alright off hte to bARS. GET LAID TONIGHT FELLAS