On Monday, at noon, in May

The rule, as people use it

These three words do the same work for time and place: they go from small and exact to big and general. "At" is a point: at six, at noon, at the door, at the bus stop. "On" is a day or a surface: on Monday, on July 4th, on the table, on the wall. "In" is a space or a longer stretch of time: in May, in 2026, in the morning, in the kitchen, in Mumbai.

For time, think of it as a funnel. "In June, on the 12th, at three o'clock." Each step gets smaller. For place, "in" means inside something with edges, "on" means touching a surface, and "at" means a point on the map where you meet or do something: "I'm at the gym." "She's at work."

A few set phrases break the pattern, and you'll hear them constantly: at night, at home, and on the weekend, where British speakers say "at the weekend". In the car but on the bus, on the train and on the plane. On the phone. In bed.

The common slip is putting a preposition where none belongs. With next, last, this, every, today, tomorrow and yesterday, there's nothing before them. So it's "I'll see you next week", not "in next week", and "We met last Friday", not "on last Friday".