As best I can remember, the followers of Christ met on the Sabbath for the services in the local synagogues, and celebrated the “Breaking of the Bread” on Sunday, the day of Christ’s Resurrection. By the time of the fall of the Temple (70? 72?AD), there had pretty much been a falling out throughout Israel between those Jews who followed Christ and those who didn’t; many, if not most had been kicked out of the Jewish services. It was not a far move from there to having a full celebration of the Word and the Eucharist on Sunday. As Christ was the fullness and the complettion fo the Law, it is easy to see that the day is easily moved from the last day (the Sabbath) to the first day (Sunday); if Christ is the fulfillment fo the Law and the Prophecies, then celebrating His day (the day of the Resurrection) is in accord with that fulfillment.