When we talk about time, we use different prepositions to introduce the time phrase.

For example:

at six o'clock
in the morning
in summer
at the weekend [British English]
on the weekend [American English]

When we talk about years, seasons, months, and periods of the day we use the preposition "in".

For example:

She got married in 1976.
Their first baby was born in summer.
He lost his job in August.
He gets out of bed in the afternoon.

To talk about days and dates, we use the preposition "on".

For example:

They were married on a Saturday.
To be precise, on June 27th, 1976.

And to talk about times or annual festivals, we use the preposition "at".

For example:

I'll see you at seven o'clock.
He always goes home at lunchtime.
They met at noon.
The party finished at midnight.
What are you doing at Christmas?
I'm visiting my mother at Easter.

There are a couple of important exceptions. Although periods of the day all use the preposition "in", as in:

in the morning
in the afternoon
in the evening

we say

at night

There is also a difference between American English and British English when we talk about "the weekend".

American English:
What are you doing on the weekend?

British English:
What are you doing at the weekend?