At + clock time
In + part of day
On + particular day
At + weekend, public holiday
In + longer period

At

With clock times we usually use at.

The meeting is at 5 pm.
I usually get up at seven o’clock.
Call me at lunch time.

In an informal style, we usually leave out at before ‘what time’.

What time does the train leave? (More natural than ‘At what time does the train leave?’)

In

We usually use in with parts of the day.

I work best in the evening.
I got up early in the morning.
I have a meeting in the afternoon.

Notes:

In the night = during one particular night

At night = during any night

On

We usually use on with days of the week and month.

He will come on Monday.
I was born on April 18th.

Public holidays

With public holidays and weekends we usually use at. For example, we say at New Year, at Christmas, at Diwali etc.

To talk about one day of the holiday, we use on.

I am visiting my grand parents on Easter Monday.

Notes:

British people usually say at the weekend; Americans usually say on the weekend.

In

In is usually used with longer periods of time.

I was born in May.
Our house was built in the 19th century.
He died in 1995.

Expressions without prepositions

Prepositions of time are not normally used before next, last, this, that, one, any, each, some, all and every.

Come any day.
See you next week.
We were up all night.