The preposition on is used in the following descriptions of time:

Prepositions of time

The preposition at is used in the following descriptions of time:

With clock times

• My last train leaves at 10:30.
• We left at midnight.
• The meeting starts at two thirty.

With specific times of day, or mealtimes

• He doesn’t like driving at night.
• I’ll go shopping at lunchtime.
• I like to read the children a story at bedtime.

With festivals

• Are you going home at Christmas/Easter?

In certain fixed expressions which refer to specific points in time

• Are you leaving at the weekend?*
• She’s working at the moment.
• He’s unavailable at present.
• I finish the course at the end of April.
• We arrived at the same time.

*Note that in American English, on the weekend is the correct form.

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The preposition in is used in the following descriptions of time:

With months, years, seasons, and longer periods of time

• I was born in 1965.
• We’re going to visit them in May.
• The pool is closed in winter.
• He was famous in the 1980’s.
• The play is set in the Middle Ages.
• They’ve done work for me in the past.

With periods of time during the day

• He’s leaving in the morning.
• She usually has a sleep in the afternoon.
• I tried to work in the evening.

To describe the amount of time needed to do something

• They managed to finish the job in two weeks.
• You can travel there and back in a day.

To indicate when something will happen in the future:

• She’ll be ready in a few minutes.
• He’s gone away but he’ll be back in a couple of days.

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The preposition on is used in the following descriptions of time:

With days of the week, and parts of days of the week

• I’ll see you on Friday.
• She usually works on Mondays.
• We’re going to the theatre on Wednesday evening.

Note that in spoken English, on is often omitted in this context, e.g. I’ll see you Friday.

With dates

• The interview is on 29th April.
• He was born on February 14th, 1995.

With special days

• She was born on Valentine’s Day.
• We move house on Christmas Eve.
• I have an exam on my birthday.

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