If what I understand is correct, the original calender stated the Sabbath was Saturday. Some Jews still practice it on this day. (At least, the ones I knew while I still lived in St. Louis.) But, since Christianity eventually grew, I think earlier on in its history, something happened to make Sunday a more conveniant day to have a Sabbath (which is a day to rest, glorify God, and meditate on his works.) All in all, though, what day of the week your Sabbath is really doesn't matter. According to Christianity, at least from what my dad has said, it's whether you practice having a Sabbath or not that matters, not which day you have it on. For instance, his Sabbath is Monday, because it is literally impossible for him to have a Sabbath on Sunday. (He is a pastor, pastors work on Sunday, obviously.)