Prepositions of time

At, on, and in

We use at for specific times.

For example:

 I start work at 7.00 a.m.

 I don't work at night.


 We use on for specific days and dates.    

For example:

 My birthday is on Monday.   

 We're having a party on 7th September.   


We also use on for some special days.   

For example:

On Christmas day.


We use in for nonspecific times during a day, a month, a season, or a year.   

For example:

In summer it's too hot to work.  

 I started this web site in 1999.

 She woke up in the night.