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.