In the Acts of the Apostles, observance of the Old Covenant sabbath was not one of the requirements enjoined on gentile Christians by the Council of Jerusalem with the words “It seemed good to the Holy Spirit and to us...”
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Act 15:19 Therefore I judge that we should not trouble those from among the Gentiles who are turning to God,
Act 15:20 but that we write to them to abstain from things polluted by idols, from sexual immorality, from things strangled, and from blood.
Act 15:21 For Moses has had throughout many generations those who preach him in every city, being read in the synagogues every Sabbath."
There was no need to tell the new converts what day the sabbath was or what day to worship on. At this point of time there was no other day known as the sabbath. The sabbath was (and still is) sundown Friday to sundown Saturday. The writers assumed that the new gentile converts would be coming to worship EVERY SABBATH. The notion that the sabbath was anything different didn't really take hold until 300 or 400 years after this and that only under threat.
Today many look at these verses through the prism of a centuries of Sunday worship and can't conceive that it was ever different.