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?
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.
- I went to London last June. (not in last June)
- He's coming back next Tuesday. (not on next Tuesday)
- I go home every Easter. (not at every Easter)
- We'll call you this evening. (not in this evening)
sources : Original Link