1. the monthly recurrence of the date of a past event: the 9th menstriversary of their relationship.
2. the celebration or commemoration of such a date.
Deriving from the Latin "mensis", meaning month, or "menstruus", meaning monthly and the Latin "vertere" meaning returning. Literally translating to: returning monthly.
Jacqueline: So, how long have you and Jillian been dating?
Justin: It's been 3 years and 8 months this past April 4th.
Jacqueline: Well, Happy 44th Menstriversary!