"Rocznica" is relating to "rok" - year
As is anniversary - anno = year in Latin.
We sometimes use 'miesięcznica' instead (miesiąc = month).
It is used in colloquial speech only and usually as a kind of joke, but it may suit in your case.
So one could say 'Wszystkiego najlepszego z okazji dziewiątej miesięcznicy (blink;-)'
Cinek