Why Terminology matters.
Sunday is never called “Sabbath” in the New Testament, but always “the first day of the week.” It is also never called “the Lord’s Day.” The only place where this term appears is Revelation 1:10. In this verse, John described that on one Lord’s day, being under the control of the Holy Spirit, he heard a magnificent voice so great that he compared it to a trumpet. The Greek term for “Lord,” kyriakē, is not a noun, but an adjective. It does not refer to a specific day of the week, such as the Sabbath or Sunday. Rather, it was a day on which John was enraptured by prophetic and divine ecstasy and received divine revelation. It was a day on which he fell under the control of the Holy Spirit and was given prophetic inspiration. Thus, for him, it was a “lordy day.”
…While the first day of the week was observed by the Jewish believers even within the land of Israel, they did not view it as a Sabbath nor were they practising “transference theology” by applying Sabbath laws to Sunday. Furthermore, nowhere in the Scriptures is the first day of the week made an obligatory day of worship; there is no command to meet on the first day of the week. It is not wrong for the local congregation to meet on the first day of the week, but it is not mandatory either. If a Jewish believer chooses to observe the Sabbath, he is free to do so, whether he observes it as a day of rest or a day of worship. If a Jewish congregation chooses to have its meeting on Saturday, it is also free to do so. However, it is forbidden to impose a mandatory Sabbath observance, individually or corporately, just as it is forbidden to mandate a Sunday observance, individually or corporately.
Dr. Arnold Fruchtenbaum, “Ariel’s Bible Commentary: The Book of Acts,” pp. 31-33.