Preposition "in, on, and at"

Prepositions of Time: at, in, on
We use:

* at for a PRECISE TIME
* in for MONTHS, YEARS, CENTURIES and LONG PERIODS
* on for DAYS and DATES

at

in

on
PRECISE TIME

MONTHS, YEARS, CENTURIES and LONG PERIODS

DAYS and DATES
at 3 o'clock

in May

on Sunday
at 10.30am

in summer

on Tuesdays
at noon

in the summer

on 6 March
at dinnertime

in 1990

on 25 Dec. 2010
at bedtime

in the 1990s

on Christmas Day
at sunrise

in the next century

on Independence Day
at sunset

in the Ice Age

on my birthday
at the moment

in the past/future

on New Year's Eve

Look at these examples:

* I have a meeting at 9am.
* The shop closes at midnight.
* Jane went home at lunchtime.
* In England, it often snows in December.
* Do you think we will go to Jupiter in the future?
* There should be a lot of progress in the next century.
* Do you work on Mondays?
* Her birthday is on 20 November.
* Where will you be on New Year's Day?

Notice the use of the preposition of time at in the following standard expressions:
Expression

Example
at night

The stars shine at night.
at the weekend

I don't usually work at the weekend.
at Christmas/Easter

I stay with my family at Christmas.
at the same time

We finished the test at the same time.
at present

He's not home at present. Try later.

Notice the use of the prepositions of time in and on in these common expressions:
in

on
in the morning

on Tuesday morning
in the mornings

on Saturday mornings
in the afternoon(s)

on Sunday afternoons
in the evening(s)

on Monday evening

When we say last, next, every, this we do not also use at, in, on.

* I went to London last June. (not in last June)
* He's coming back next Tuesday. (not on next Tuesday)
* I go home every Easter. (not at every Easter)
* We'll call you this evening. (not in this evening)