on
on
in
in
at主要表示：
(1)在某具体时刻之前,如at seven o’ clock,at 7：30.
(2)在固定短语中,如：at noon,at night,at that time,at the age of at the weekend,at Christmas.
On用来表示“在……天”,如：On Monday,On May lst,On Children’s Day.
in用来表示：
(1)在某年、某月、某季节.
(2)在—段时间之后,如：in two hours,in a few days.
注意：在纯粹地表示在上午／下午／晚上时,用in the morning／afternoon／evening,但在某一天的上午、下午、晚上前要用介词on.如：on Monday morning,on the morning of Children’ s Day.