This misunderstanding is probably where the idea that Monday is the first day of the week came from. Biblically, Saturday is the Sabbath, the last day of the week. But early Christians came together before and after work on the first day of the week, the day Jesus rose from the dead, which is explicitly the day after tue Sabbath. Eventually that day was also made a free day amd added to the weekend. But it was always the first day of the week until ISO redefined it.