They were keeping the 7th day holy for thousands of years at the specific commandment of God who created the 7th day holy:
Gen 2:3 And God blessed the seventh day, and sanctified it: because that in it he had rested from all his work which God created and made.
You see, God created the 7th day holy at the beginning of the world. He codified it in the ten commandments. Israel and all God fearing non-Israelites kept the day holy. At the time of the apostles there was never a thought, inkling, or idea that it wasn't holy. There was no need to sit down and figure out what day was set aside by God almighty. Everyone knew. It was read in the scriptures every sabbath. Jesus Christ, God almighty on earth, kept it as an example. You would have had to be an illiterate caveman NOT to know that the 7th day was holy and set aside by God.
I would recommend doing a historical study on the evolution of the sabbath in Christianity because history is where it happened, not scripture.
You didn't answer my question. I take it you don't have an answer.
Canon XXIX...."Christians must not judaize by resting on the Sabbath, but must work on that day, rather honouring the Lord's Day; and, if they can, resting then as Christians. But if any shall be found to be judaizers, let them be anathema from Christ."
39 years earlier "The Council of Nicea" laid the groundwork for much anti-semitic legislation of later church councils. The Council of Antioch (341 A.D.) prohibited Christians from celebrating Passover with the Jews. The first mention of "The Venerable Day of the Sun" was made at Nicea which had been convened by Constantine, a (former) Sun Worshiper".
These new laws and canons were of course now backed up by civil law and military power. Don't you think it strange that it took over 300 years to get out the word? Those pesky Christian judaizers....I swear!
You know....300 years ago is a long time. We still had not invented the "Pet Rock" 300 years ago.