1.he is going to telephone at five o'clock.["在几点几分"一律用at]
2.My birthday is on May 21st.[“在某月某日”用on(因为是具体的一天)]
3.It is always cold in February.[“在某月”用in]
4.My father was there in 1984.[“在某年”用in]
5.he is going to arrive on Tuesday.[“在某星期”用on]