In the United States, at least, the first day of the "work" week for most of us is Monday, yet most, if not all, of our calendars start the week on Sunday. Why is that? A quick search on Google turned up "Calendars Through the Ages," which gives the following explanation:

"The Bible clearly makes the Sabbath the last day of the week, but does not share how that corresponds to our 7 day week. Yet through extra-biblical sources it is possible to determine that the Sabbath at the time of Christ corresponds to our current 'Saturday.' Therefore it is common Jewish and Christian practice to regard Sunday as the first day of the week (as is also evident from the Portuguese names for the week days). However, the fact that, for example, Russian uses the name "second" for Tuesday, indicates that some nations regard Monday as the first day.

In international standard ISO-8601 the International Organization for Standardization (ISO) has decreed that Monday shall be the first day of the week."