[bliss]

I've always thought of this as the Jewish calendar, where Monday start is the Christian calendar. All based upon the day on which God rested. So if Sunday is the sabbath, then Monday is the start of the week. Whereas if Saturday is the sabbath, then Sunday is the start. (For the purposes of balance, I'm aware other religions exist too)

[lotharbot]

Saturday is still the day on which God rested in the Christian calendar. The significance of Sunday comes from Jesus' resurrection. It wasn't until the Middle Ages that Sunday came to be treated as a "rest day" and therefore conflated with the idea of a "Sabbath" (see http://en.wikipedia.org/wiki/Lord%27s_Day ). Even then, Sunday was generally termed the "first day" -- note that "Seventh Day Adventists" hold their religious services on Saturday.