A day in a school, usually Middle or High school, where teenagers go around slapping each others butts for the fun of it. Common for a guy to slap a girl's ass, a girl to slap a guy's, or two female friends to slap each other, but is usually taboo for two guys to do so. Normally occurs on Fridays and the message is often passed through a forwarded text.
FWD: Fwd: Friday is Slap-ass day! forward to all members of *name random high or middle school* in your contact list!