I made the comment that the Jewish Christians probably met as Jews in the synagogue on Saturday-Day, and when six pm rolled around, the Gentile Christians joined them, and the meeting morphed into a Christian assembly as the first day of the week began at the close of the Sabbath, our Saturday-Night.
The idea that Jewish Christians were still meeting in the synagogue as Jews didn't seem to sit well with some. This is probably because many Christians today believe that keeping the Mosaic Law after Jesus' resurrection is sinful, based on a faulty understanding of Paul's writings that the Law has been done away with.
Paul didn't say that keeping the Law was sinful; he said that trying to be justified by keeping the Law was sinful. He further said that the Law was good.
We have traditionally misunderstood these warnings in the New Testament against adhering to the Mosaic Law. It is not a sin to keep the Law; it is a sin to seek justification by keeping the Law. This misunderstanding has led to the further misunderstanding of much of the history of the church in the first century.
Paul, himself, kept the Law. From Acts 21:
21[The Jews] have been informed that you teach all the Jews who live among the Gentiles to turn away from Moses, telling them not to circumcise their children or live according to our customs. ... so do what we tell you. ... Then everybody will know there is no truth in these reports about you, but that you yourself are living in obedience to the law..."
... 26The next day Paul [did as they suggested].
Not only Paul, but the entire church kept the Law for the first decade or so of the church. The whole reason for the Jerusalem conference in Acts 15 is because up until then, every Christian was a Jew keeping the Law of Moses; it likely had never even crossed their minds that Gentiles could become Christians before Peter met Cornelius, much less that they could do so without "becoming Jews" and keeping the Law of Moses. The entire conference was about this issue, and even the Apostles and other leaders of the church had to be swayed by the Holy Spirit before they'd accept the idea that the Law of Moses was not required of the Gentile converts (although they insisted that the Gentiles keep a few basics to keep from offending the Jews living in Gentile cities). The New Testament has several examples of Law-keeping Jewish Christians still trying years later to coerce the Gentile Christians to keep the Law of Moses, particularly circumcision. (Those Jewish Christians must not have gotten the memo; or they just disagreed with it.)
Modern-day Christians, if they truly want to be Biblical, need to realize that in the first decade or so of the church Christians were exclusively Moses-obeying Jews, and then for the next few decades there were "two" churches : a Moses-obeying Jewish church, and a non-Moses-obeying Gentile church. Paul had to constantly remind the Christians that they had been made into "one" by the blood of Christ which made this distinction a merely cultural one that no longer mattered. Thus he could add that keeping a Jewish holy day special didn't matter one way or the other (Rom 14:5ff); Jewish dietary restrictions didn't matter one way or the other (Rom 14:6ff); Jewish circumcision didn't matter one way or the other (1 Cor 7:19). Once the Jews got that into their heads, they either started letting go of Moses, or started letting go of Christ in order to hold onto Moses, and the result was a gradual morphing of the church into a predominantly non-Moses-obeying Gentile church, which is what we have today.
This lesson is hard to accept for modern Christians because it brings into question doctrines many of us have held all our lives, such as our thinking about observing religious holidays. But our job is not to hold onto our life-long-held doctrines, but to examine the Scriptures daily to see if these things be so.