In my catechism it asked;"What day is the Sabbath?
A: Saturday
Q: Why do we worship on Sunday instead of Saturday?
A: Because the Holy Catholic Church changed the day from Saturday to Sunday.
Q: By what authority did the church change the day from Saturday to Sunday?
A: By the plenitude of that divine power bestowed on her by Jesus Christ.
So there you pretty much have it.
I think you have a false dichotomy or false dilemma here, other than the replacing of the Old Covenant of the Law with the New Covenant of Grace (Hebrews Chapters 8-10).
I don't see where the Sabbath was necessarily the day of Jewish worship before Christ. The Sabbath was simply a day of rest for the Jews.
After Christ's resurrection, Jesus himself appeared to his disciples on Sundays until he ascended to Heaven. So Sundays became a day of corporate worship and also a day of rest. There's nothing in the Bible that says Saturday's Sabbath was done away except the law that REQUIRED you to rest on Saturday was abolished (Heb. 8:13).
And OBTW, the authority for individual or church action should ultimately be God's Word, not man's organizational decrees.
For Jews, Sabbath begins at sundown on Friday.
Jesus was discovered to have risen from the dead on Sunday morning, so I’d say that’s why Sunday is the day of rest for Christians.