The re-occurrence of a holiday, regardless of similar date to previous years, that holds a special meaning to a person because one of the previous celebrations of this holiday involved a major event (i.e. first date, starting a friendship, losing virginity, moving); if the holiday moves, than the "holiversary" moves too
"Dude I'm jewish and don't have any plans for easter, wanna hang out?"
"sorry man, me and my girlfriend are celebrating our holiversary since we met volunteering at a neighborhood easter-egg hunt last year."
"OK, I know we're just friends with benefits, but it's been one year since we first started this whole thing and I wanted to take you out to dinner to celebrate. Happy Humpiversary!"