An important day, very much like an anniversary
, but it is celebrated in terms of months, as opposed to anniversaries which are celebrated in terms of years. The only difference is that there are only 11 monthsaries for a certain event, the 12th would already be an anniversary.
If a couple got together on April 18, their monthsary, rather, monthsaries, would be on May 18, June 18, July 18, ..., until March 18 of the following year. Then, The next 18th, which is April, would be logically, their first anniversary