A day that occurs near your birthday (usually within 3 or 4 days of it) that really sucks ass for no good reason. Some believe it is meant to balance out the good day that you had on your birthday. Often, an anti-birthday will occur even if your birthday wasn't that great.
John:"Hey Jim! I heard u turned 17 the other day!"
Jim:"ya..." *walks away*
John:"what's with him?"
Joe:"oh dude he's havin his anti-birthday. he's been depressed all day"