Which day is originally the first day of a week?

A recent report has shown that here in the United States, we’ve experienced an evolution concerning our attitudes towards the workweek and the weekends. Although some calendars still mark the beginning of a week as Sunday, more and more of us are coming to regard Monday as the first day of the week with Saturday and Sunday comprising the two-day period thought as the weekend. In fact the word "weekend" didn’t even exist in English until about the middle of the 19th century. In England at that time, Saturday afternoons had just been added to Sundays and holidays as a time for workers to have off from their jobs. This innovation became common in the United States in the 1920s, but as the workweek were shortened during the Great Depression of the 1930s, the weekend expanded to two full days — Saturday and Sunday. Some people thought that this trend would continue due to increasing automation and the workweek might decrease to four days or even fewer. But so far this hasn’t happened. The workweek seems to have stabilized as forty hours made up of five eight-hour days.