Prepositions of Time

There are three main prepositions of time in English: at, on and in

1.    We use at to talk about a specific time or age:

At six o’clock (6 a.m. / p.m.)
At midnight (12 a.m.)
At the age of 16
At quarter past three
At midday

2.    We use on to talk about a specific day or date:

On the 25th December
On the 16th May
On Sunday mornings
On Monday

3.    We use in to talk about a period of time:

In spring / summer / autumn / winter
In January / February
In the morning / the afternoon, the evening (BUT, at night)
In 1990, 2001 etc.