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. (noton next Tuesday)
I go home every Easter. (not at every Easter)
We'll call you this evening. (not in this evening)

For a full list of 150 prepositions, including one-word and complex prepositions, with 370 example sentences, try the English Club ebook English Prepositions Listed.