Most of my life I wondered why it's called "middle of the week" when actually Thursday's the day with an equal number of days in a week coming before and after. I often thought maybe weekend's subtracted. Only in my late 20s I learned that there are places where the first day of the week is Sunday lol
I'm swiss and I always assumed its because of the workweek being Monday to Friday. But only a few decades ago Saturday was pretty mich a workday as well, so that probably isnt it
More directly, we can't agree if Sunday or Monday is the first day. IMO Sunday is the first day. Calendars look better with the weekends acting like bookends.