Actually, they probably picked Sunday because the Jews had Saturday and the Christian church was a subset of Judaism for a while. They were still good Jews on Saturday and celebrated as Christians on Sunday. That’s why there was a debate about allowing Gentiles into the church.
By that time, the Law had been fulfilled. The church was teaching that dietary laws and circumcision rules were no longer required. Sunday was kept because they really didn’t see any Scriptural requirement for the actual physical day. It should just be the Seventh Day.
The SDA criticism of the Catholic Church is likely misplaced. The Eastern Orthodox Church is probably the real perpetrator.
Source: Codex Justinianus, lib. 3, tit. 12, 3; trans. in Philip Schaff, History of the Christian Church, Vol. 3 (5th ed.; New York: Scribner, 1902), p. 380, note 1. First Sunday Law enacted by Emperor Constantine - March, 321 A.D.
shalom b'SHEM Yah'shua HaMashiach On the venerable Day of the Sun let the magistrates and people
Followers of Yah'shua always met on His commanded Sabbath.
residing in cities rest, and let all workshops be closed. In the country,
however, persons engaged in agriculture may freely and lawfully
continue their pursuits; because it often happens that another day is
not so suitable for grain-sowing or for vine-planting; lest by neglecting
the proper moment for such operations the bounty of heaven should be lost.
(Given the 7th day of March, Crispus and Constantine being consuls each of them for the second time [A.D. 321].)