mensiversary is the most often suggested formulation for this, with luniversary a (poor) second choice. neither has any grounding in history (i.e., both are neologisms).

medieval times did give us a related term, month's mind marked the 30th day after a Christian's death or burial.

edit: Merriam-Webster, in its descriptive way, gives:
broadly : a date that follows such an event by a specified period of time measured in units other than years <the 6-month anniversary of the accident>

Last edited by tsuwm; 05/14/2008 4:58 AM.