Shouldn't the translation be "the guests are coming ON Sunday"?
Technically, yes, but I would say that "the guests are coming Sunday" is also informally correct.
I think "ON Sunday" is more typical in UK English and just "Sunday" in US English. Correct me if I am wrong! (I'm British and I say "ON Sunday".)
So I think both answers should definitely be accepted.
I'm American, and I have heard and would say either.
"ON" is missing.
'The guests arrive on Sunday' is apparently wrong. Eish!
Coming and arriving are 2 different verbs with 2 different meanings