In English, the prepositions "in", "on" and "at" are frequently used to express dates.
Use of 'in, on, at' with Days
'On' is used when talking about days of the week and special days such as birthdays.
I saw him on Monday.
Our wedding is on the 15th of May.
We went to the beach on my birthday.
We're having a party on New Year's Eve.
We usually visit our grandparents on Sundays.
'In' is used when talking about a large part of the day.
(in the morning, in the afternoon, in the evening)
He studies in the evening.
I didn't drink coffee in the morning.
'At' is used when talking about a specific part of the day.
(at noon, at night, at midnight, at dawn)
I'll call you at noon.
It’s hard to see stars at night in the city.
When talking about a part of a particular day, 'on' is used, not 'in' or 'at'.
(on Monday night, on Friday afternoon, on Sunday morning)
We will meet on Tuesday evening.
I have a meeting on Thursday morning.
Use of 'in, on, at' with Months
'In' is used when talking about months.
Summer starts in June.
'On' is used when talking about a particular day in the month.
Christmas is on December 25th.
She moved to a new house on the first of January.
Use of 'in, on, at' with Years
'In' is used when talking about years, decades or centuries.
She was born in 1995.
In the 1990s, global communication became much easier.
In the next century, technology will be even more advanced.
'At' is used to talk about the beginning or end of a day, month or year.
I will call you at the end of the day.
Let’s meet at the beginning of the week.
We are planning a vacation at the end of May.
They will move to a new house at the end of the month.
Others
'In' is used for seasons.
They go skiing in winter.
'At' is used when talking about precise specific times.
The train leaves at 8:30 AM.
I’ll see you at lunchtime.
The meeting is on Wednesday at 2 PM.
'In' is used when talking about the past and future.
In the past, people used to write letters.
In the future, we might travel to Mars.
When talking about the weekend, we use 'on' in American English and 'at' in British English.
She’ll be back at the weekend.
We will go to the beach on the weekend.
When talking about an event or celebration, 'at' is used, and when talking about the day of the event, 'on' is used.
I always relax on Christmas Day.
They always celebrate at Christmas.
What did you wear at my wedding?
They arrived on my wedding day.
'In' is used when talking about a specific period of time.
I’ll finish the report in an hour.
They will move to a new house in a few months.