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

AT

Use "at" with precise time:

§ at six o"clock

§ at 10.30

§ at two p.m.

ON

Use "on" with days of the week:

§ on Monday

§ on Fridays

Use "on" with specific calendar days:

§ on Christmas day

§ on October 22nd

IMPORTANT NOTES

in the morning / afternoon / evening - at night

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