The preposition at is used in the following descriptions of time:

With clock times

• My last train leaves at 10:30.
• We left at midnight.
• The meeting starts at two thirty.

With specific times of day, or mealtimes

• He doesn’t like driving at night.
• I’ll go shopping at lunchtime.
• I like to read the children a story at bedtime.

With festivals

• Are you going home at Christmas/Easter?

In certain fixed expressions which refer to specific points in time

• Are you leaving at the weekend?*
• She’s working at the moment.
• He’s unavailable at present.
• I finish the course at the end of April.
• We arrived at the same time.

*Note that in American English, on the weekend is the correct form.

The preposition in is used in the following descriptions of time:

With months, years, seasons, and longer periods of time

• I was born in 1965.
• We’re going to visit them in May.
• The pool is closed in winter.
• He was famous in the 1980’s.
• The play is set in the Middle Ages.
• They’ve done work for me in the past.

With periods of time during the day

• He’s leaving in the morning.
• She usually has a sleep in the afternoon.
• I tried to work in the evening.

To describe the amount of time needed to do something

• They managed to finish the job in two weeks.
• You can travel there and back in a day.

To indicate when something will happen in the future:

• She’ll be ready in a few minutes.
• He’s gone away but he’ll be back in a couple of days.

The preposition on is used in the following descriptions of time:

With days of the week, and parts of days of the week

• I’ll see you on Friday.
• She usually works on Mondays.
• We’re going to the theatre on Wednesday evening.

Note that in spoken English, on is often omitted in this context, e.g. I’ll see you Friday.

With dates

• The interview is on 29th April.
• He was born on February 14th, 1995.

With special days

• She was born on Valentine’s Day.
• We move house on Christmas Eve.
• I have an exam on my birthday.

If we examine these different aspects of usage for the three prepositions, a general pattern emerges. At is generally used in reference to specific times on the clock or points of time in the day. In generally refers to longer periods of time, several hours or more. On is used with dates and named days of the week.

http://www.onestopenglish.com/grammar/grammar-reference/nouns-and-phrases/prepositions-of-time-and-place-article/152825.article