What day is it? 主要用于問“星期幾”;What's the date? 用于問日期。之所以不用which,是因為在問話人心目中,他(她)并沒有明確的選擇范圍。如果有明確的選擇范圍用which也是可能的。如:
On which day are you leaving, Monday or Tuesday?
On which date are you leaving, July 10 or July 11?