1. "At" is used for time and certain expressions:
- at 8.00
- at lunchtime
- at Christmas
- at the moment
- at the weekend
- at the same time
- at the age of 5
2. "On" is used for days and dates:
- on Friday
- on 12 September
- on Friday morning
3. "In" - for longer periods such as months, years, seasons, etc.:
- in January
- in 2012
- in summer
- in the 20th century