Not being religious, I'm also curious--is it actually written down in the Bible that Sunday is the sabbath? Because Sunday is the first day of the week, not the seventh, according to every single calendar I've ever seen.
The word sabbath means "rest," so it need not be on the seventh day. The Bible records that Christ's resurrection occurred on the first day of the week, and that the early Christians often met together on the first day, which we now call Sunday. Presumably, though, many of these Jewish converts would have continued to meet in the synagogue on the seventh day. As Christians, especially Gentile converts, no longer observed the civil and ceremonial aspects of the Jewish Law, Sunday services with baptisms, communion, hymns and preaching supplanted the Jewish Sabbath.
Christians in America today, however, are pretty sketchy on the idea of a day of rest. Many consider it a sort of ritual to go out for Sunday dinner at a restaurant (and letting others work for you would also violate the Sabbath). A lot of us think nothing of working Sunday, and going to church Saturday night, instead, or of churching Sunday morning and going out to see a movie that night (making others work for you again). Then there are folks like me who go to church in the morning, eat dinner at home, sit around all day, and then go back for an evening service. Don't work or buy stuff on Sunday unless in dire need. It definitely ain't observing the Sabbath in the Jewish sense, as my wife cooks elaborate meals every Sunday afternoon, and I do the dishes.
I have seen calenders, though, in which Monday comes first as it's the first day of the business week.