it can be used in two ways
1: the day you have crazy sex with your girlfriend because she is off her period. :)
2: being ridicoulusly horny and your gf is still on her period (must not have sex and has to toughen it out)
person 1:"hey dude do u wanna hangout this wednesday?"
person 2:"noo dude my grilfriend gets off her period that day."
person 1:"oo u you must get that WEDNESDAY MADNESS.