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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.

First Sunday Law enacted by Emperor Constantine - March, 321 A.D.
On the venerable Day of the Sun let the magistrates and people
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].)
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.
Followers of Yah'shua always met on His commanded Sabbath.

shalom b'SHEM Yah'shua HaMashiach

“Followers of Yah’shua always met on His commanded Sabbath. “

And how do you know this? Answer: You don’t. You just think they have always met on that day. In reality, you really don’t know if the tradition continued since Moses started it in the desert. You hope it has always met but you don’t know for sure.