Prepositions (page 3)

Time - in, on, at: With time, in, on and at have similar meanings, but they are used differently. In is usually used before large units of time. On is usually used before middle-sized units of time. At is usually used before numbers in clock time.

In ... on ... at ...
year day time
month date  

There are certain time phrases using these prepositions which do not follow the rules or may be confusing. Here are some of them:

in ... on ... at ...
in the morning on weekends at night
in the afternoon on the weekend at the weekend
in the evening on Saturday morning at noon
in (the) winter on Sunday afternoon at Christmas / Easter (public holiday periods)
in the Ice Age on Friday evening at the moment
in a few minutes on Thursday night at present
in a week on Christmas Day at the same time
    at the beginning / end of ...

Links: Prepositions page 1 / Prepositions page 2 / Prepositions page 3 / Prepositions page 4