anniversary for months
Whether anniversary can mark a span shorter than a year.
Strictly, an anniversary is a yearly return of a date. Prefer six months, the six-month mark, or six months to the day when the span is not a year. Six-month anniversary and monthly anniversary are widely understood and common in casual use; they are loose. Do not build forms such as monthiversary.
Examples
Use They marked six months of the project on Tuesday.
Prefer the six-month mark
Avoid their monthiversary
Greeting-card English will keep six-month anniversary. In our usage dictionary, the yearly sense is the default.