"What day is it tomorrow?"
几号 is the date (1st, 2nd, 3rd,...25th, etc.) The question is about the day of the week (monday, tuesday, etc.)
I typed 明天几月几号 and it showed incorrect because the question is about day of the week, and not the date.
明 = next 天 = day (明天= tomorrow) 星= small amount/ star 期= a period of time (星期 = day of the week) 几= how many