At, in and on are all prepositions of time.
We use at to talk about specific times.
I will meet you there at 6 o’clock.
He met her at midnight.
She is not feeling well at the moment.
We use in to talk about longer periods of time: months, years, centuries and so on.
Her birthday is in January.
He was born in the year 1903.
They were some of the coolest people in the 15th century.
We use on to talk about specific days and dates.
I will see you on Friday.
His appointment is on March 12th.
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