The Christian observes the first day of the week in a different way than the Jews kept the Sabbath in the Old Testament. The first day of the week, called, “The Lord’s Day,” is a day when saints fellowship together in obedience to the instruction of the writer of Hebrews, where he said: “Let us hold fast the profession of our faith without wavering;… not forsaking the assembling of ourselves together, as the manner of some is; but exhorting one another.” (Hebrews 10:23, 25). The Sabbath is a state of daily Christian walk; The Lord’s day is the day when saints fellowship together on the first day of the week.
Do you really suppose that the writer of Hebrews had the thought in his mind that Christians should fellowship together on the first day of the week when he wrote Hebrews 10:23, 25? I submit to you that he had no such thought whatsoever, for Sunday keeping had not arisen in the Christian church until many years later.
Is there any proof whatsoever in the Bible that “the Lord’s day” is the first day of the week? The term “the Lord’s day” is only used once in the Bible, in Revelation 1:10. John wrote, “I was in the Spirit on the Lord’s day, and heard behind me a great voice, as of a trumpet.” (Revelation 1:10) There is no indication in the Bible that this term was applied to the first day of the week. In fact the term “first day of the week” is used 8 times in the Bible, 6 of which are used in the gospels for the day of Christ’s resurrection. The remaining 2 times it is used do not indicate that it is the Lord’s day. In fact the only day of the week that could possibly be referred to as “the Lord’s day,” is “the seventh day,” which “is the sabbath of the LORD thy God.” (Exodus 20:10) Jesus also said, “the Son of man is Lord even of the sabbath day.” (Matthew 12:8) The seventh-day Sabbath is “the Lord’s day” according to the Scriptures.
I hope this helps to clarify the issue of the law and the Sabbath in the New Testament.