AT, IN, ON (time)

AT
PARTICULAR POINTS	ON THE CLOCK	I’ll see you at five o’clock.
	IN THE DAY	The helicopter took off at midday and headed for the island.
	IN THE WEEK	What are you doing at the weekend? UK What are you doing on the weekend? USA
WITH SPECIAL CELEBRATIONS		At the New Year, millions of people travel home to be with their families (but we say on your birthday).

ON
WITH DATES	We moved into this house on 25 October 1987.
WITH A SINGULAR DAY OF THE WEEK TO REFER TO ONE OCCASION	I’ve got to go to London on Friday.
WITH A PLURAL DAY OF THE WEEK TO REFER TO REPEATED EVENTS	The office is closed on Fridays. (every Friday) In informal situations, we often leave out on before plural days: Do you work Saturdays?
WITH SPECIAL DATES	What do you normally do on your birthday?

IN
WITH PARTS OF THE DAY	I’ll come and see you in the morning for a cup of coffee, okay?
WITH MONTHS	We usually go camping in July or August.
WITH YEARS	The house was built in 1835
WITH SEASONS	The garden is wonderful in the spring when all the flowers come out.
WITH LONG PERIODS OF TIME	The population of Europe doubled in the nineteenth century.