“Is Peter's party on a Sunday?” Why is the indefinite article used?

Question

I saw this sentence in a primary school English textbook:

Is Peter's party on a Sunday?

I wondered why they put an "a" before Sunday. The teacher said it was correct but didn't explain.

Could someone please explain?

Answer 1

In the question, the emphasis is on what day of the week the party is.

If the question were,"Is Peter's party on Sunday?", it would imply the coming Sunday. The party is presumably on a later date and the emphasis of the question is on what day of the week the party is.

Is it a Sunday, as opposed to any other day?