at表示“在……时刻”,与具体的钟点搭配使用,如：
We open at seven and close at ten.
We start work at 8 o’clock
on表示“在……时候”,与日期、星期搭配使用.如：
I leave on Monday 25th November.
He plays football on Friday.
*in表示“在……期间”,与具体的年代、月份、季节或一天中的某个时段搭配使用.如：
The postman comes in the morning(s) and in the afternoon(s).邮递员每天上午和下午来送信.
注意：on Monday与on Mondays意义不完全相同.on Monday有两个意思,第一表示特指的某个星期一,第二表示每逢星期一；而on Mondays只表示每逢星期一.
注意:具体某天的上午、下午或晚上要用介词on,如：
They will have dinner together on Sunday evening.
*注意:以下名词与介词的固定搭配：
My children play in the street in the daytime. (在白天)
I can hear the cars at night. (在晚上)
I go to my mother’s house at the weekend. (在周末)
I go to my mother’s house on the weekend.(美式英语)
I play tennis at weekends. (每逢周末)