Earlier quoted context omitted.
The world of finance is full of anachronisms like this. Exchanges are also closed on various bank holidays which are different in every country.
> Exchanges are also closed on various bank holidays which are different in every country You can't settle on a bank holiday, because the banks are closed. If you trade on a day you can't settle, you're going to have two days of trading settling on the same day later, which is going to be weird.
For example, if there’s a US holiday on a Friday then EUR/USD trades conducted on the Wednesday and Thursday would both typically settle Monday.