Prepositions: time (basic)

Questions
Overview

The most common time prepositions are in, on and at:

In

category example
years In 1966
decades In the 1980's
centuries In the 21st century
periods of the day* in the morning
seasons in the summer
future periods in two weeks' time
months in June

Exception: both 'in the night' and 'at night' are used depending on the situation

On

category example
days of the week on Tuesday
parts of days on Tuesday afternoon
dates on July 21st
specific days on Christmas day

At

category example
exact clock times at 4pm, at noon
meals at lunch (time)
festival periods at Christmas

It is important to remember that there are many situations where no preposition is used, e.g. I will see you on tomorrow.
Further learning
Description Author Language
How to use 'on' when expressing time. Smrt English
How to use 'in' when expressing time. Smrt English
How to use 'at' when expressing time. Smrt English
5 situations where 'in' can be used to express time. BBC Learning
How to use 'in' with months, seasons, years and times of day. BBC Learning