The standard western week is Saturday-Sunday. Who says so? The several Western languages that refer to Saturday with words that mean "Sabbath" or "seventh day"

The standard western week is Saturday-Sunday. Who says so? The several Western languages that refer to Saturday with words that mean "Sabbath" or "seventh day" suggest that at least part of the world thinks that Sunday is the first day of the week, and Saturday the last. (I am aware that Chinese treats Saturday as the sixth day of the week, and didn't have a concept of seven-day weeks before trade with the West.) There isn't universal agreement on this, is the short answer to your question.