We use:
• at for a PRECISE TIME
• in for MONTHS, YEARS, CENTURIES and LONG PERIODS
• on for DAYS and DATES
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:
Notice the use of the prepositions of time in and on in these common expressions:
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)
http://www.englishvideolesson.com/2170-preposition-at-english-grammar-online.html
http://www.englishvideolesson.com/2169-preposition-on-english-grammar-online.html
http://www.englishvideolesson.com/2168-preposition-of-place-in-english-grammar-online.html
