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...	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

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

Expression
at night
at the weekend
at Christmas/Easter
at the same time
at present

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