I found it odd that this question immediately spawn religious reasons for the first day of the week. For me, it has nothing to do with beliefs, but with work schedule and social word usage.
Work starts on monday and you always work for the weekend. Saturday and Sunday are always considered the weekend, thus Sunday would be the last day at the end of the week. Monday starts the new week.
But for me, I work five day shifts, so my week start is always different. My work week always starts on a wednesday, friday, or monday. But football is on the weekends, especially on Sunday, so I vote for monday to be the first day of the week.
We were taught that Sunday is the first day of the week, but as Konquererz pointed out, it's still the weekend, as in the end of the week, so I have always considered for me personally that Monday is the first day of my week.
I wouldn't work a Sunday anyway, so my first day of the week would naturally start on a Monday..but that's just how it works for me.