@Ceili,
Not to nitpick, and it really isn't that important. Christians in the Bible didn't celebrate the Sabbath on Sunday. They celebrated on Saturday.
Remember that the Christians in the Bible were Jewish.They knew and believed the story of God creating the world in six days and resting on the seventh day. This is why in Moses set up the Sabbath (the seventh day) as holy. The seventh day was holy to Christians and Jews alike.
The Christians all know that the Sabbath (the day commanded by God in the law of Moses to keep holy) was the seventh day. However the Christians started meeting 0n the "First Day" (they referred to it as the first day) to break bread.
They were two different days and early Christians, being Jewish, celebrated both days. There was no confusion over which day was referred to in the law of Moses.
The word Sabbath is a Jewish word that comes from the law of Moses which says "Remember the Sabbath and keep it Holy". There is no question that this refers to the seventh day (on which God rested) and calling Sunday the Sabbath is just wrong. The Biblical Christians would have understood that.
The fact they met on Sunday makes sense, it it doesn't make it the Sabbath.