The monthly recurring date of a past event, especially one of historical, national, or personal importance. Often referred to as a "month anniversary." This is wrong, because "anniversary" comes from the root "ano" meaning year, while "menoversary" comes from "meno" meaning month.
I asked my girlfriend out on October 24th, 2005. Our 1st menoversary is on Thanksgiving, and our 2nd menoversary will be on Christmas Eve.