How to name a date that is half a year after the date of an important or special event?

An anniversary is

a date that is an exact number of years after the date of an important or special event

I'm wondering if there are some words indicating a date that is half a year, a fortnight or even a month after the date of an important or special event.

Given:

semi-, prefix: half of or occurring halfway through (a specified period of time) [MW]

A semi-anniversary (or semianniversary) would be a six-month anniversary.

(Edit: The term di-anniversary shows up in searches; the only problem is, di- means "twice : twofold : double" not "half.")

Some Googling turns up mensiversary (from Latin mensis for month) as a term for a one-month anniversary.

If you tortured Latin sufficiently, you could probably formulate a term for a fortnight anniversary; but then the language police would get you.

There had been some attempts in trying to promote the use of certain neologisms, but none of them apparently really caught on. The safest way of referring to such dates is by using the simpler term anniversary preceded by the period.

I wouldn't personally use any other alternative. Imagine that if they can't agree on a basic period like a month, what is left for fortnights or weeks or any other less common arbitrary period of time.

According to Merriam-Webster:

Definition of ANNIVERSARY
1 : the annual recurrence of a date marking a notable event;
broadly : a date that follows such an event by a specified period of time measured in units other than years

Hope it helps.