Are you free on that day为什么要加on?
问题描述：
Are you free on that day为什么要加on?
day前面有其他词不是不用加介词了吗?
答
具体某一天前要用on
星期前用on
on指时间表示：
（1）具体的时日和一个特定的时间,如某日、某节日、星期几等.如：
On Christmas Day（On May 4th）,there will be a celebration.
（2）在某个特定的早晨、下午或晚上.如：
He arrived at 10 o’clock on the night of the 5th.
（3）准时,按时.如：
If the train should be on time,I should reach home before dark.
4.by指时间表示：
（1）不迟于,在（某时）前.如：
He will come by six o’clock.
Jack had made some friends by the time you came.
（2）在……间,在……的时候.如：
He worked by day and slept by night.
5.through指时间意为“从……开始到结束”,此时与throughout相同.如：
We work hard all through the year.