a corruption of anniversary, "anniversarius" latin for "annus" (year) + "versarius" (to go back to or revisit).
1. "month" in latin is mensis. thus, it should be "mensiversary", or menses for brevity, since anybody who celebrates this thing, more likely than not, is having his/her monthly period.
2. since "month" is named after the moon (luna in spanish), then it is very safe indeed to consider that anyone who celebrates this event is a lunatic.
2. Girl: Honey, it's that time of the month again!
Guy: (To himself) Oh crap, just great! I just bought a couple of viagras and now this. I'm going out for a beer.
Girl: What are you talking about? It's our monthsary!
Guy: (has already ran out the door) ...
DICK 2: ITS MONTHIVERSARY sucka