More likely, it goes back to the origins of our 7-day week. The ancient Romans had an 8-day week. Our 7-day week comes from the Jewish calendar, where suppos

More likely, it goes back to the origins of our 7-day week. The ancient Romans had an 8-day week. Our 7-day week comes from the Jewish calendar, where supposedly the world was created on a Sunday, and the final day of the week is the Jewish Shabbat on Saturday. (Though, we didn't keep the Jewish calendar's notion that days start at sunset.) I suspect that if you go back far enough, most of Europe considered the week to begin on Sunday, but I have no idea how far you'd have to go back.