n 1: The monthly recurring date of a past event, especially one of historical, national, or personal importance: a relationship monthaversary (monthiversary); the monthaversary of ones birthday (bithday).
2: A celebration commemorating such a date.
n: the date on which an event occurred in some previous month (or the celebration of it)
In dumb down stupid terms: A monthly anniversary, though usually only used in context for the first 12 months
Today is the 2nd monthaversary of that nasty hurricane that hit two months ago