In On At

at 3 o'clock

at noon

at bedtime

in May

in summer

in 1990

on Sundayon

on Tuesdays

on 6 March

on Christmas Day

I have a meeting at 9am.

Jane went home at lunchtime.

In England, it often snows in December.

Do you think we will go to Jupiter in the future?

At:  tells us that the following noun is located at a specific point or location. It shows an exact position.

In: tells us the noun is in an enclosed space (surround or closed off on all sides). Basically, when something is inside something.

On: tells us that the following noun is located on a surface. Use on when one thing is attatched to or touching somthing.

"She's waiting at the entrance."

"He's sitting on his chair at his desk."

"I work at a bank."

In a box.

In a room.

In a country.

On the table.

On the floor.

On the chair.