The Day after Mardi Gras that many Juniors and Seniors in High School dread, or don't even go school, due to the Hangover symptoms they have gotten from the previous day.
Guy1: dude, where were you? We had a baseball gameyesterday!
Guy2: sorry man, yesterday was Hangover Wednesday, and i just had to celebrate it.
Similar to Ash Wednesday, where you get a cross wiped on your forehead, Ass Wednesday is where you insert two fingers into your partner's ass, then smear a poop cross on their forehead. Must be performed on Wednesdays.