at refers to the time as timepoint during the day or during the season or during the year.

Let's meet at 3PM.

We were there at Easter.

They make a special offering to their deities at planting time.

on refers to the time as occasion.

The parade is on New Year's Day.

in refers to the time as timespan.

I expect to see them in the next week or two.

at refers to the time as timepoint during the day or during the month or during the season or during a calendar year.

Let's meet at 3PM.

We were there at Easter.

They perform this ceremony at the new moon.

They make a special offering to their deities at planting time.

on refers to the time as occasion.

The parade is on New Year's Day.

The parade is on Easter Day.

The convention will be held on June 15th.

in refers to the time as timespan.

I expect to see them in the next week or two.

In this new electric kettle water comes to a boil in only a few minutes.