In Jewish-Christian tradition, Sunday has always been the first day of the week, and Saturday the seventh. No church (or anyone else) has ever changed this. (Of course bussinesses and other groups may organize work calenders according to their own convenience, much as bussinesses and other groups use a “fiscal year” that may or may not begin on January 1.) What has changed is what is considered the primary holy day. Jews, since the beginning of time (literally, according to tradition) have worshipped on Saturday (beginning at sundown on Friday evening). Saturday is the Sabbath (literally, “seventh”), the seventh day of creation, and the day of rest prescribed by the Ten Commandments. No church has ever denied this (AFAIK).
The very first Christians were also Jews. They would go to synagogue on Saturday with all the other Jews in the community, and then on Sunday, the first day, would gather by themselves to celebrate the Ressurection of the Lord, which biblical accounts describe as taking place “on the first day of the week.” As time passed and Christianity and Judaism diverged, Christian Jews began to find themselves unwelcome in synagogues. Gentiles, whom pretty much everyone agreed were free from obligations and tradtions under the Jewish law, increasingly joined the church, and may never have worshipped on Saturday, not being Jews. Furthermore, some, like St. Paul, began arguing that even Jewish Christians should avoid following the Jewish law in ways that might seem to seperate them from Gentile Christians or implied an obligation to that law. The net result of all these things was that within a hudred years or so, the church, which was now was almost entirely Gentile anyway, worshipped only on Sunday (in the morning, usually, when Christ was presumably raised).
“Sabbath” has often been used by Christians as a name for Sunday, but it was either by analogy with the true sabbath (seventh) as a traditional day of rest and worship, or from confusion. Sunday is traditionally called “the Lord’s Day” in the church, although it’s not something you hear in conversation. (Even church bullitins usually just call it “Sunday” anymore.) Some, such as Friends (Quakers), still call Sunday “First Day.” (Though this, too, is seen as a little affected by most Quakers I know.) Interestingly, the early church also called Sunday “the Eighth Day”! Like the eighth note in a musical octave, Sunday functioned simutaneously as the first in a new series and the last in an old series. It was the final completion of the order of creation, which had taken place in the first seven days, and the begining of the new creation. Cool, huh?
Seventh Day Adventists (and a few other Seventh Day churches) don’t disagree with the rest of Christianity on the ordering of the calender, only on the authority of the early church to disregard Jewish law by changing the day of worship.
