At, on and in (time)

출처: English Grammar Today

We use at:

with particular points on the clock:

I’ll see you at five o’clock.

with particular points in the day:

The helicopter took off at midday and headed for the island.

with particular points in the week:

What are you doing at the weekend?

with special celebrations:

At the New Year, millions of people travel home to be with their families (but we say on your birthday).

We don’t use at with the question What time …? in informal situations:

What time are you leaving? (preferred to At what time are you leaving?)

We use on:

with dates:

We moved into this house on 25 October 1987.

with a singular day of the week to refer to one occasion:

I’ve got to go to London on Friday.

with a plural day of the week to refer to repeated events:

The office is closed on Fridays. (every Friday) In informal situations, we often leave out on before plural days:

Do you work Saturdays?

with special dates:

What do you normally do on your birthday?

We use in:

with parts of the day:

I’ll come and see you in the morning for a cup of coffee, okay?

with months:

We usually go camping in July or August.

with years:

The house was built in 1835.

with seasons:

The garden is wonderful in the spring when all the flowers come out.

with long periods of time:

The population of Europe doubled in the nineteenth century.

At or on?

We use at to talk about public holidays and weekends, but when we talk about a particular special day or weekend, we use on.

Compare

We never go away at the New Year* because the traffic is awful.*	On New Year’s Day, the whole family gets together.
I’ll go and see my mother at the weekend* if the weather’s okay.*	The folk festival is always held on the last weekend in July.

*Note that American English speakers usually say on the weekend.

In or on?

We use in with morning, afternoon, evening and night, but we use on when we talk about a specific morning, afternoon, etc., or when we describe the part of the day.

Compare

I always work best in the morning. I often get tired in the afternoon.	The ship left the harbour on the morning of the ninth of November.
In the evening they used to sit outside and watch the sun going down.	It happened on a beautiful summer’s evening.

At or in?

In the night usually refers to one particular night; at night refers to any night in general:

I was awake in the night, thinking about all the things that have happened.

‘It’s not safe to travel at night,’ the officer said.

At the end or in the end?

We use at the end (often with of) to talk about the point in time where something finishes. We use in the end to talk about things that happen after a long time or after a series of other events:

At the end of the film, everyone was crying.

Not: ~~In the end of the film~~ …

I looked everywhere for the book but couldn’t find it, so in the end I bought a new copy.

At the beginning or in the beginning?

We use at the beginning (often with of) to talk about the point where something starts. We usually use in the beginning when we contrast two situations in time:

At the beginning of every lesson, the teacher told the children a little story.

In the beginning, nobody understood what was happening, but after she explained everything very carefully, things were much clearer.

Other uses of in with time

We use in to say how long it takes someone to do something:

He was such a clever musician. He could learn a song in about five minutes.

We use an apostrophe -s construction (in a year’s time, in two months’ time) to say when something will happen. We don’t use it to say how long someone takes to do something: