We use At with times :

at 5 o’clock ,  at 11:45   , at midnight , at lunchtime
Carol usually leaves work at five o’clock.
At night ==> I don’t like going out at night.
At Christmas / at Easter ==> They give each other presents at Christmas.
At the moment / at present ==> Ann and I arrive at the moment / at present.

At the same time ==> Ann and I arrived at the same time.
At the age of ==> Tom left school at the age of 16.
At the beginning of ==> I’m going away at the beginning of May.
At the end of ==>At the end of concert, there was great applause.

We use ON with dates and days
on March 12th   , on Friday ( s ) , on Christmas day  ( at Christmas )
On Friday morning (s ) , on Sunday afternoon (s) , on Monday evening (s) , on Saturday night (s) , on weekend
For example :
I usually go out on Monday evenings.
What are you doing on the weekend?
We use in for longer periods of time ( month, year, season)
in April
in 1968
in winter
in the 18th  century
in the 1970s
in the morning
in the afternoon
in the evening
They got married in 1968.
She went to Bali in winter.
I’ll see you in the morning.

IN + period of time = a time in the future
The train will be leaving in a few minutes. ( = a few minutes from now )
Jack went away. He’ll be back in a week. ( a week from now )
They are getting married in six months. ( six months from now )
you can say “ in six months’ time” , “in a week’s time”, etc


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