"What day is it tomorrow?"
几 is used as a question word in "What date?" and "What day of the week?" The answers to questions in Chinese always just replace the question word with the answer. So in 星期几, you answer 星期___ and replace 几 with the day of the week. With 几号, your answer is a number and then 号 which means date. If I ask 今天几号? you would answer "20号". The order of when 几 is in front or behind is just learned, there's not really a definite rule that I know of. Hope this helps!
Would it make sense to change the English question to "What day of the week is tomorrow?"? From my understanding in Chinese it is clear, that 明天星期几 asks for the day of the week and 明天几月几号 asks for the date. However, the answer to the English question "What day is it tomorrow?" could be either the date or the day of the week.