It was explained to me that the reason for Sunday worship is that Saturday/Sabbath was considered too holy for even the church leaders/priests to work, so Sunday became the day of worship after a day of rest.
I’m not saying this is what happened, but it is an explanation I received once. It sounds plausible to me, but I’m so gullible...
I'll bet that the BOOK might give us a clue...
Acts 20:7
And upon the first day of the week, when the disciples came together to break bread, Paul preached unto them, ready to depart on the morrow; and continued his speech until midnight.