I answered the same, but I guess it means what day will it be tomorrow.

Yes, I have the same answer and I don't get why it's wrong

It is possible, but without the dash and also less common.

I think the most grammatical version of the eng. sentence is "which day is it..." It is confusing though that you need to translate "what" rather than "which"

Который is a relative clause word, as in «Мой дом, который находится в Москве» ("My house which is in Moscow"). Какой is a question word, like asking "which one?".

Какой - which (one)

Что - what

If you can use "which (one)" instead of "what" in a Russian sentence, use it. Native Russians, please correct me if I'm wrong.