Why do we use “on” for days?

English speakers say “on Monday”, but why do we use “on” for days?

We say:

  • “on Friday” – a day of the week
  • “on my birthday” – a specific day
  • “on March 7th” – a specific date

In, on, and at – place and time

In, on and at are often used to say when things happen. With in and at the connection between place and time is quite clear.

For in:

  • “in the living room”: a room has walls, it starts at one wall and ends at the other. The next room is on the other side of the wall.
  • “in 2021”: a year has a start and end: it starts on January 1st and ends on December 31st. The next year comes after that.

For at:

  • “at the train station”: at refers to a point in space.
  • “at 12:50”: at refers to a point in time.

On is a little more abstract.

  • “on the table”: the surface of the table doesn’t have a clear start and end.
  • “on Sunday”: Sunday doesn’t have a clear start and end. (My day does not start and end at midnight…)

When we look at objects we see the surface.

If we look at an apple, where does the surface of the apple start? It doesn’t have a clear start and end.

Think of the Earth. When we map it, we start with whichever place is most important for us and put it in the middle if we can.

We can put Europe and Africa in the middle.

We can put the Pacific in the middle.

Both maps show the surface of the Earth. There is no clear place where it has to start or end. This idea applies to the surface of any physical object.

Time is difficult because we can’t see time. But we can think about it in the same way.

When does a day start and end?

Now with digital clocks, the answer is 12:00 midnight. But the English language is older than this. The way we speak is based on people.

Think of a day. It is not clear when someone is going to start and end their day.

  • Some people start their day early and finish early: 4:00 AM – 9:00 PM
  • Others start and finish later: 10:00 AM – 1:00 AM. (Notice that even though they have gone past midnight, it is still one day, not two)

There is not time when someone’s day must start and end.

So, on fits well. And when we speak, on feels natural.

In everyday English, we say “on Monday” very often. Using “on” for days just feels right. But, the basic meaning of on in this situation is the same as in any situation.

Uses of “on” are based on this idea: a surface – something that doesn’t have a clear start or end.

We also use on in other ways, on still has the same meaning:

Put a hat on. (Cover the surface of your head)
Come on! (Don’t stop now – no end yet!)
Last night Brian went on and on about American politics. (It seemed like there was no end)

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