Well ... if you go by the beliefs of many Christians, that they worship on the day god rested after creation ... and they hold services on Sunday ... then Monday would be the first day of the week, because Sunday is the last.
Of course, then there are Seventh Day Adventists, who worship on Saturday because they believe that is the last day of the week - so Sunday is the first.
Then again, the ancient Etruscans had an 8-day week (nundinum). So do the Burmese Buddhists. Though they "cheat" by dividing Wednesday into midnight to noon, and noon to midnight.
Personally, I think of Sunday as the first day simply because that's what most calendars show.
It depends on the purpose. Sunday is the first day of the calendar week, but many governments and employers consider Monday the first day, with the payroll period being Monday-Sunday.
Monday if you consider it to be Sunday. Sunday if you consider it to be Saturday.
Most Calendars show Sunday as the first day of the week. I have always considered Sun they first day which was confusing when I read about the Sabbath being the last day of the week.
I still consider Sunday the first day. And Saturday as the true Sabbath. I believe the church moved it long ago as way to separate from the Jewish church.
According to the calendar, Sunday is the first day of the week. Saturday is the last day of the week. That’s why we call Saturday and Sunday the weekend. (Don’t get me started on “hump day”.)
Sunday. The calendar refill I bought last year had the weeks' first day as Monday. I could not get used to it and missed a couple of appointments. I bought a new genuine Franklin Covey refill mid year because of it. I will not try that again even though the price was 40% less that the authentic one. It's hard to say that, being Thrifty and all................