Germany switched to starting Monday in 1969/1976. Before, since the middle age, it had been practice to start the week on Sunday. However, I don't know why this was changed.
Traditionally, the seventh, "no-work", day is Sabbat, which happens to be on Saturday.
Wouldn't the day God rested have been the first full day humanity existed, and so the first day of the week (and of all time). I don't really use a particular day to start my week personally, I keep my calendars in day by day mode.