Why ziua a opta and not opta ziua?

So "din luna ianuarie" and "din luna lui ianuarie" both work (were accepted). Any difference in style or usage?

Why do we need the „a” before „lunii” here?

Can I not say 'a opta ziua'? If so why not (as in what is the rule)?

I just had word tiles and luna was not available - i tried din ianuarie and it wouldn't accept it but i don't know what else i could have done.

Why is it: "ziua a opta"??? I didn't see this before in this Duolingo course....