Please provide some context.Do you capitalize the word anniversary?
I like to think of it as something special, so I treat it as a proper noun. Just my style
Rereading this, Language Hound is right---context would clear this up.Please provide some context.
Do you plan to use the word in a sentence? As part of a greeting?
It is sometimes capitalized and sometimes not (most usually not I would say).
Without context, impossible to say.
It's a common noun; there's no justification for capping it—unless, as has been noted, it's part of a greeting (as Language Hound says in post #5).
The same is true of birthday.