But what about Acts 20:7?I think you would be partly correct in the above...the dynamics of the change took place over several centuries, many influences came into play. The Jewish uprising against Rome and Rome's easy recognition of Jews because of their Sabbath keeping....the Catholic church's change from passover to easter (one of the many pagan compromises in the early church, including the trinity and Sunday sacredness) after which Sunday keeping was emphasized...and papal Rome's hatred of the Sabbath because it was a sign of rejection of papal authority.
As for what day Jesus rose and what day He entered Jerusalem and what day Pentecost...all essentially irrelevant. Once Jesus died...once His blood confirmed the covenant and no word from Him had changed anything, then that sealed the deal. No apostle, no church, no council then had the right to change the testament. Any change was a usurpation of authority.
I believe the first day of the week is Sunday. It sounds like the disciples were gathered together for communion which would be church - the breaking of bread.
New American Standard Bible
On the first day of the week, when we were gathered together to break bread, Paul began talking to them, intending to leave the next day, and he prolonged his message until midnight.