The subject and the verb have to agree in grammatical number.
Since the subject remains
Sunday, the verb remains
works (3rd-person singular present). Why it's ok to use the present tense to talk about the future is actually a lot more complicated, but you didn't ask about that.
The months are objects of the preposition
in and have no effect on the verb. Screwing up subject-verb agreement because of intervening prepositional phrases is a common mistake among native speakers but doesn't have any special name as far as I know.
(It's worth mentioning that some prepositional phrases do change the grammatical number of the subject. For example, 'Some of us are...' but 'Some of the money is...' Your sentence doesn't involve the words 'some', 'any', 'none', 'all', or 'most', though, so that side rule doesn't come into play for you.)