People often say it, but I just don’t believe it. It just sounds like something people say to seem smart (“well actually…”) whenever you mention (or imply) that the week begins on a Monday. I’m talking about Western civilization here, by the way.
First of all, in practice, Monday is definitely the first day of the week.
Secondly, Sunday is one of two days that make up what we call “the weekend”.
Lastly, Sunday surely equals the “seventh day” on which God rested, if you are to believe the Bible, which was presumably a big influence on the structure of our weeks.
The “seventh day” from the Bible is what we know as Saturday. That’s probably the beginning and end of this question, since the “first day” terminology comes from there as well.
Christians go to church on Sunday to commemorate the resurrection of Jesus. Yes, it gets called the ‘Sabbath’, and that might actually influence the idea of Monday as the first day. Jews still celebrate the Sabbath as a day of rest on Saturday.
The payweek at my work is Sunday to Saturday. Most calendars in the US show Sunday at the far left. When they don’t, it’s very confusing. There really isn’t an official answer to what day the week starts on.
Last I checked, the Hebrew calendar says you are wrong. The seventh day is essentially the Gregorian Saturday, since Hebrew days run from night to night. In Israel, your workweek begins on Sunday.
I’ve lived with both systems, and in my opinion, it really doesn’t matter whether you say the first day of the week is Sunday, Monday, or Thursday. What matters is that everyone agrees.
Biblically yes, as evidenced above. Practically, a society can just pick a day randomly–I hear that in European countries, calendars start on Monday. Scientifically, some cosmologist might be able to give the day (immediately after) the big bang happened, pretending the Earth and Sun already existed, as they do for years. If calculations can be that precise. (Dating from the formation of Earth and Sun wold also work, but it would require variable length years and days.)
I suppose, different countries have different traditions here. Some international web communities which provide you with a calendar for your profile page, give you the choice between Monday and Sunday as the first day of the week.
Since the day is the only unit of time that has any real significance in terms of its duration, even though the day itself is not quite as regular as nit-pickers would like, the arbitrariness of the week can be traced to a more-or-less correspondence with the phases of the moon, and the month itself to a rough approximation of the number of days/nights between successive phases.
So whenever you choose to start counting off seven successive days and calling that group a week, you have no natural constraints on what that group should mean. Convention and habit are all you can appeal to for “what’s right” in that exercise, and one’s as good as another as long as you don’t disagree with others who think of it one way and you want it to be another.
More specifically, I believe, the Jewish Sabbath begins on Friday evening at sunset. So, when it’s after dark on Friday evening on the Gregorian calendar, the seventh Jewish day has already begun. The seventh day in the Gregorian calendar won’t start until midnight. This means that the Sabbath effectively ends at sunset on Gregorian Saturday, so (Gregorian) Saturday Evenings are part of the regular Jewish week.