Steve Burton (Right Reason) writes: "100th anniversary". That's precisely the right way to put it, as opposed to the wrong but increasingly common ways, such as "five-year anniversary" or, more barbarically, "six-month anniversary."
The word "anniversary" means "the annually recurring date of a past event." To write or say "x-year anniversary" is redundant as well as graceless. To write or say "x-month anniversary" is nonsensical; what is meant is that such-and-such happened "x" months ago.