"What day is it tomorrow?"
Translation:Какой завтра день?
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.