IN

Use 'in' months and years and periods of time:

in January

in 1978

in the twenties

Use 'in' a period of time in the future:

in a few weeks

in a couple of days

ON

Use 'on' with days of the week:

on Monday

on Fridays

Use 'on' with specific calendar days:

on Christmas day

on October 22nd

AT

Use 'at' with precise time:

at six o'clock

at 10.30

at two p.m.

at lunchtime / dinnertime

IMPORTANT NOTES

We say in the morning, afternoon or evening BUT we say 'at night'