Prepositions of Time: at, in, on

We use:

at
PRECISE TIME
in
MONTHS, YEARS, CENTURIES and LONG PERIODS
on
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:

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.

*Note that in some varieties of English people say "on the weekend" and "on Christmas".

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 afternoon(s)
in the evening(s) on Monday evening(s)

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

See also Prepositions of Place: at, in, on