Consider the import of what Paul says here. He is to be a light to the Gentiles, showing them the way. What day did he appoint to meet with these people? He had a wonderful opportunity at the end of that first Sabbath, which broke up in a dispute, to tell the people: "Now, wait a minute. Yes, I want you to all come back, but why not just do it tomorrow, on Sunday?" Instead, in order to continue in the grace of God, he had them wait a full week. Then, on the Sabbath, they assembled again. Note that this occurs in the general area of Galatia.