下面提供一些 in 和 on 的对于时间和日期的习惯用法：
IN
Use 'in' months and years and periods of time:
in January
in 1978
in the twenties
Use 'in' a period of time in the future:
in a few weeks
in a couple of days
AT
Use 'at' with precise time:
at six o'clock
at 10.30
at two p.m.
ON
Use 'on' with days of the week:
on Monday
on Fridays
Use 'on' with specific calendar days:
on Christmas day
on October 22nd
注意：
我们说
in the morning / afternoon / evening
但我们说
on the morning / afternoon / evening of
而且,我们说
at night
就你的提问：
前天早晨 / 前天下午
the morning / afternoon of the day before yesterday 前面加 in 或 on 皆可,
但本人的习惯是用 on
前天晚上9点半的时候 / 后天早上9点半的时候
用 on 和 in 都可以：
Your flight departed at half past nine (on/in) the evening of the day before yesterday
The flight took place (on/in) the evening of the day before yesterday
但两者语境不同,
用 on 来表示“在那一天” ,比如 the day before yesterday 前天
用 in 来表示“时间”,比如“傍晚时间”,during the hours of the evening,即 6点整到 10 点整左右.
说实话,很多英语母语者对这个英语的诡异也很纠结,
就是这个原因,一般人忽略 in 和 on 介词,
干脆只用名词 morning ,afternoon 等等：
Your flight departed at half past nine the evening of the day before yesterday
The flight took place the evening of the day before yesterday