While it is true that the word anniversary is rooted in the Latin word for "year", there isn't a word that I know of to denote an occasion marking a lesser period. I guess in former times people only noted a Jahreszeit, although we do have centennials, and fractions and multiples of centennials. Since no one is going to talk about a menseversary, or a bihebdoversary etc., I guess we'll just have to go on using anniversary as an all-purpose word with suitable modifiers for the time span.

Postscript
Egad, Faldage got in mensiversary while I was writing the above.