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