We use:
1. at for a PRECISE TIME
2. in for MONTHS, YEARS, CENTURIES and LONG PERIODS
3. on for DAYS and DATES

Example:
1. We have a meeting at 9 am.
2. The office closes at midnight.
3. In Iceland, it often snows in December.
4. Do you think we will go to Mars in the future?
5. Do you work on weekend?
6. His Anniversary is on 21 November.