We use:
1. at for a PRECISE TIME
2. in for MONTHS, YEARS, CENTURIES and LONG PERIODS
3. on for DAYS and DATES
Examples:
1. I have a meeting at 9am.
2. The shop closes at midnight.
3. In England, it often snows in December.
4. Do you think we will go to Jupiter in the future?
5. Do you work on Mondays?
6. Her birthday is on 20 November.

When we say last, next, every, this we do not also use at, in, on.
1. I went to London last June. (not in last June)
2. He's coming back next Tuesday. (not on next Tuesday)
3. I go home every Easter. (not at every Easter)
4. We'll call you this evening. (not in this evening)