What's the appropriate term for a non-annual commemoration of an event?

"remembrance" or "memorial" are reasonable, though each has a connotation of honoring something or someone no longer with us.
I've heard "a celebration" of X used for more happy events.

‘Anniversary’ itself seems to be changing its meaning so as to describe the marking of an event at intervals of other than twelve months.

How about mensinary, from the Latin root mensis, "month"?

“Nth mensiversary” is (rarely) used for a N-month commemoration.

My suggestion is to call such an event a "commemoration", as "commemoration" doesn't necessarily mean this takes place yearly:

an observance or celebration designed to honor the memory of some person or event.