We print calendars that put Sunday as the first day of the week, but in practice we begin our week on Monday, and (also) call Sunday the second day of the weekend, meaning that for all practical purposes we work from Monday to either Friday or Saturday, then take Sunday off, resting on the seventh day of our week. How we handle Sunday then becomes the issue.
Well, I do get your point.
The problem for that argument is that the people who became the Jews were the ancestors of people who had kept the sabbath since Eden.
I don’t believe authority figures really argue that Sunday is the seventh day of the week, with conviction.
Eons ago, there was a calendar change to compensate for leap years not observed. I believe they added ten days onto the calendar, but the days of the week were not disturbed.
For instance, a Monday wasn’t jumped ahead or back to a Thursday. (as an example (only)). The days were jumped ahead as in the 5th to the 15th. (again just an example)
I don’t recall the exact days involved. The particulars concerning the days of the week should be sound.