We use:
- at for a PRECISE TIME
- in for MONTHS, YEARS, CENTURIES and LONG PERIODS
- on for DAYS and DATES
| 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:
- 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?
See more at:
https://www.englishclub.com/grammar/prepositions-at-in-on-time.htm