As stangdon mentioned in the comments, the missing verb (and its tense) is meant to be inferred from context.
If the phrase was referring to the past, or to the present, it would still be written as A wonderful month for me.
It might help to consider that this isn't really a complete sentence. It's more of an interjection/exclamation.
The equivalent full sentence would be:
It will be a wonderful month for me!
The subject here is "it", and the verb is "will be".
If the context were in the past, the sentence would be:
It was a wonderful month for me!
And if it were in the present:
It is a wonderful month for me!
The entire "it is/will be/was" part, the subject and the verb, is dropped in all cases.
Grammatically, this construction is analogous to using a single adjective as an interjection, like "Good!" or "Nice!" or "Nasty!" or "Cool!".