This post starts with the most common use of the three prepositions, followed by other usages. Wherever possible, the prepositions have been presented in tabular form for ease of comprehension. Comments are in square brackets in blue font so that you can easily spot them. And the post ends in a quiz for you to test yourself.

Let’s start with the most common use of the three prepositions.

1. The most common use of At, On, and In

AtOnIn
It is used with exact point of time.It is used for a particular day, date, or part of the day.It is used for longer periods such as months, seasons, years, decades, and centuries.
Examples
At 4 PM [Exact time]On Thursday [Particular day]In 1986 [A long period]
At 3 o’clockOn Thursday morning [Part of the day]In the 20th century
At sunrise/ sunsetOn Christmas/ Independence Day [Particular day]In 1970s
At midnightOn 06 April 2016In April
At noonOn his birthdayIn summer

2. Other use of At

With short holiday periods:

Examples

We often go hiking in the nearby forest at the weekend. [However, ‘on the weekend’ is more often spoken in common conversation]

What are you doing at Christmas? [But: what are you doing on Christmas Day? ‘Christmas’ and ‘Christmas Day’ are different]

I’ll see you at Easter. [But: I’ll see you on Easter Day.]

With other short periods that are treated as points:

Examples

My salary gets credited at the end of every month.

I’m due for promotion at the end of January.

at the beginning of the year.

With mealtimes:

Examples

I’ll see you at the breakfast.

Let’s discuss the business plan at lunchtime.

At dinner.

With night when we mean ‘when it is night’ or ‘each night’:

Examples

With the power to street lights cut off because of non-payment of electricity bills, it’s difficult to go out on the streets at night. [It means ‘each night’ and not one or two nights]

I got up in the middle of the night after a bad dream. [At is not used here because the reference is to a particular night]

3. Other use of On

We say on time to convey that someone or something is not late:

Examples

Please be on time for the meeting.

The flight is on time.

We say on the dot to convey that something happened at the exact time specified:

Examples

The meeting started on the dot at 11 AM.

The train will leave at 6 AM on the dot.

4. Other use of In

With parts of the day:

Examples

In the morning [But: on Monday morning. Note that ‘Monday morning’ is part of the day (refer to examples in section 1)]

In the afternoon [But: on Monday afternoon]

In the evening [But: on Monday evening]

When we talk about how long it will be before something happens:

Examples

We’ll be leaving in few minutes.

She’ll be here in a moment.

He has gone to his home town and will be back in a week.

When we say how long something takes:

Examples

He learnt driving in just a week.

5. More…

We don’t use at, on, or in before all/ any/ each/ every/ last/ next/ one/ some/ this/ that/ today/ tomorrow/ day after tomorrow/ soon/ later:

Examples

He is leaving the country next March. [Not: in next March]

I’ll see you next Thursday. [Not: on next Thursday. But: I’ll see you on Thursday]

Let’s meet tomorrow. [Not: on tomorrow]

I was in Madrid last Monday. [Not: on last Monday]

Quiz

Fill in the blanks with the preposition of time at, on, or in.

1. Many medical treatments with no scientific basis originated ___ the middle ages.

2. Polar bears hibernate ___ winter.

3. X: What are planning ___ Easter Day? Y: Nothing much. Though I may go out of the town for two days.

4. I’ll see you ___ noon in your office.

5. I got more presents than I expected ___ my birthday.

6. Some of the best music was composed ___ 1980s.

7. Will you be in the town ___ Christmas?

8. I’m planning a trip to my hometown ___ the end of April.

9. Can we meet ___ Monday evening?

10. Can we meet ___ the evening today?

1. in   2. in/ during   3. on   4. at   5. on   6. in   7. at   8. at   9. on   10. in