WCF 21.1
The light of nature showeth that there is a God, who hath lordship and sovereignty over all; is good, and doth good unto all; and is therefore to be feared, loved, praised, called upon, trusted in, and served, with all the heart, and with all the soul, and with all the might. But the acceptable way of worshipping the true God is instituted by Himself, and so limited by His own revealed will, that He may not be worshipped according to the imaginations and devices of men, or the suggestions of Satan, under any visible representation, or any other way not prescribed in the Holy Scripture.
WCF 21.7
As it is the law of nature, that, in general, a due proportion of time be set apart for the worship of God; so, in His Word, by a positive, moral, and perpetual commandment binding all men in all ages, He hath particularly appointed one day in seven, for a Sabbath, to be kept holy unto Him: which, from the beginning of the world to the resurrection of Christ, was the last day of the week; and, from the resurrection of Christ, was changed into the first day of the week, which, in Scripture, is called the Lord’s Day, and is to be continued to the end of the world, as the Christian Sabbath.
From Joseph A. Pipa, Jr.:
“With respect to the Sabbath, the moral requirement is for a regularly recurring amount of time to be allocated exclusively to God’s worship. Specifying a day or a period of time is a moral positive law. The changing of the latter does not affect the character of the former. Therefore, God may and indeed has changed the day. The Westminster Confession alludes to this when it refers to the Sabbath as “a positive, moral, and perpetual commandment”:
“The light of nature showeth that there is a God, who hath lordship and sovereignty over all; is good, and doth good unto all; and is therefore to be feared, loved, praised, called upon, trusted in, and served, with all the heart, and with all the soul, and with all the might… As it is the law of nature, that, in general, a due proportion of time be set apart for the worship of God; so, in His Word, by a positive, moral, and perpetual commandment binding all men in all ages, He hath particularly appointed one day in seven, for a Sabbath…”
The Sabbath ordinance contained the positive law of one whole day in seven; that, from the creation, was the seventh day. But the day may be changed without affecting the inherent moral character of the ordinance.
We ought not to think of the Sabbath therefore as a temporary law, destined to pass away with the Mosaic covenant. Rather, as Dabney asserts:
“The reason that the ceremonial laws were temporary was that the necessity for them was temporary. They were abrogated because they were no longer needed. But the practical need for a Sabbath is the same in all ages. When it is made to appear that this day is the bulwark of practical religion in the world, that its proper observance everywhere goes hand in hand with piety and the true worship of God; that where there is no Sabbath there is no Christianity, it becomes an impossible supposition that God would make the institution temporary. The necessity for the Sabbath has not ceased, therefore it is not abrogated. In its nature, as well as its necessity, it is a permanent, moral command. All such laws are as incapable of change as the God in whose character they are founded. Unlike mere positive or ceremonial ordinances, the authority of which ceases as soon as God sees fit to repeal the command for them, moral precepts can never be repealed; because the purpose to repeal them would imply a change in the unchangeable, and a depravation in the perfect character of God.””
“The Lord’s Day” by Joseph A. Pipa, jr.
-pages 35-36