English uses prepositions of time before many time expressions. The main prepositions are in, on, and at. They show if the time is general, more specific, or very exact.

Use in for months, years, seasons, and parts of the day. In shows a general time around a period, not one exact point.

Rule
Use in with months.
Use in with years.
Use in with seasons.
Use in with parts of the day.

Use on for days and dates. On shows a more specific time than in.

Rule
Use on with days of the week.
Use on with day plus part of the day.
Use on with dates.

Use at for clock times and specific moments. At shows one exact time point.

Rule
Use at with clock times.
Use at with exact moments such as noon, midnight, and night.
Use at with fixed expressions of time.

Use in for long time periods, on for days and dates, and at for exact times. This is the basic order from general to exact.

Word or Phrase	Definition
in	It is used for a general period of time.
on	It is used for a specific day or date.
at	It is used for an exact time or moment.

Some holidays use different prepositions. Use on with a holiday when you mean the holiday day. Use at with holiday periods such as at Christmas or at Easter. Use in with holiday months or years.

Rule
Use on for the holiday day.
Use at for holiday periods.
Use in for the month or year of a holiday.

You can now choose in, on, and at for many time expressions. You can talk about months, years, seasons, parts of the day, days, dates, clock times, holidays, and fixed moments. You can say time in a clear and correct way.