Is there a reason for the days of the week being written and pronounced without the 日? [duplicate]

Question

I sometimes see the days of the week written without 日. Are there specific reasons on when 日would not be used?

Answer 1

That's a informal way of saying.

For example:

月曜にまた会おう

This sentence gives listeners a friendly-impression. So you can't say 月曜, 火曜, 水曜, … to your boss.