不加.为什么？on 只针对某个具体日子，如on May 1st, on Monday等，且这个日子前没有定语。next Monday， next是定语，这种情况也不加。所以He is playing tennis with me on tomorrow是错的？He is playing tennis with metomorrow是对的吗？是错的！后面的正确。 但He is going to play tennis with metomorrow.更好些