[typetheorist]

A lot of people have already commented on how Israel and the Middle East consider Sunday to be the first day of the week, but I'd like to add that the week itself seems to originate in Judaism (Wikipedia, "Week"):

"A continuous seven-day cycle that runs throughout history without reference to the phases of the moon was first practiced in Judaism, dated to the 6th century BC at the latest."

In Judaism the week starts from Sunday, so you could argue that it's not completely illogical for it to be the case in the US.

Changing the rest day of the week to Sunday was a change made in Christianity by the Council of Laodicea:

"Christians must not judaize by resting on the Sabbath, but must work on that day, rather honouring the Lord’s Day; and, if they can, resting then as Christians. But if any shall be found to be judaizers, let them be anathema from Christ".