Here’s what Garner’s Modern American Usage has to say about anniversary:
anniversary (= the day of the year on which an event occurred in a previous year) is today used informally to denote a milestone in months or even weeks. That usage has become increasingly common, perhaps because there is no convenient equivalent for terms shorter than a year (milestone is close, but it doesn’t connote observance and recurrence the way anniversary does). Considering the word’s tight association with “year,” however, the loose usage is subject to criticism and should be avoided if possible.
Drafters are prone to this loose use of anniversary—”month anniversary” occurs in 2,818 contracts filed on the SEC’s EDGAR system as material contracts during the past year. (That’s a lot.) Eliminating this usage results in prose that is not only more logical but also more concise. Here’s an example:
The Company shall redeem this debenture in 24 equal installments of principal and accrued interest monthly beginning on the one (1) month anniversary following [read one month after] the First Closing Date.
But in terms of boneheadedness, I don’t think you can beat 12-month anniversary, as in ending on the 12-month anniversary of that termination date. The adjectival phrase 12-month is redundant!