We use on with days and weekend(American English).
on Saturday morning
on the 1st of October.
on the weekend (American English)
I got up early on Tuesday morning.
My school starts on the 21st of November.
We use at with night, weekend(British English) and an exact time.
at lunch time
at the weekend
Dominos is working at night.
She goes to the cinema at the weekend.
My lesson starts at 9 o’clock.
We use in with: years, seasons, months, centuries, morning, evening and afternoon.
in the 21st century
in the 1990s
in the summer
in the evening