men·stri·ver·sa·ry
/mɛnstrəˈvɜrsəri/

pronunciation men-struh-vur-suh-ree
–noun

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.

Etymology:
Deriving from the Latin "mensis", meaning month, or "menstruus", meaning monthly and the Latin "vertere" meaning returning. Literally translating to: returning monthly.