grammatical number - What is the "s" at the end of some words and where does it go when talking about multiples?

I just recently wrote a sentence to a client, then had to correct the sentence and wondered about one of the words.

I wrote the sentence:

Any Sunday in September works

Then I realized I could also do any Sunday in October, so I had to change the sentence to

Any Sunday in September & October work

Where did the "s" go in "works"? What is that called and what are the rules?

Answer

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.)