Chapter 24 of the “Live Forever” book published in 1982 has the most thorough explanation as to why Christians are no longer under the Law, including the Sabbath, (and is the only WT publication that I’ve ever seen that has a whole chapter on the subject). Some points have already been covered here, but the scriptural reasoning in paragraphs 7-9 are what cinched it for me…
Some persons, however, say that the Law is divided into two parts: The Ten Commandments, and the rest of the laws. The rest of the laws, they say, are what ended, but the Ten Commandments remain. Yet this is not true. In his Sermon on the Mount Jesus quoted from the Ten Commandments as well as other parts of the Law and made no distinction between them. Jesus thus showed that the law of Moses was not divided into two parts.-Matthew 5:21-42.
Notice, too, what the apostle Paul was inspired by God to write: “Now we have been discharged from the Law.” Was it only the laws other than the Ten Commandments that the Jews were discharged from? No, for Paul goes on to say: “Really I would not have come to know sin if it had not been for the Law; and, for example, I would not have known covetousness if the Law had not said: ‘You must not covet.'” (Romans 7: 6, 7; Exodus 20:17) Since “You must not covet is the last one of the Ten Commandments, it follows that the Israelites were discharged from the Ten Commandments also.
Does this mean that the law to keep a weekly Sabbath, which is the fourth of the Ten Commandments, was also removed? Yes, it does. What the Bible says at Galatians 48-11 and Colossians 216, 17 shows that Christians are not under God’s law given to the Israelites, with its requirement to keep the weekly Sabbath and to observe other special days in the year. That keeping a weekly Sabbath is not a Christian requirement can also be seen from Romans 14:5