This is from an article by John Murray published in 1941. The whole article is worth reading.
The Sanctity of the Day
First, and most elementally and centrally, it is that one day in seven is distinguished from the other six. That day is to be sanctified, and at the heart of the word “sanctify is the idea of distinction and separation. This one day is set off, it is placed in a distinct category. This import of the word cannot be evaded and it is to be very carefully marked, for on it depends the whole notion of what we may and must call the “sanctity” of the Sabbath.
It is not, however, the bare notion of distinction or separation that is expressed in the commandment. The command to sanctify occurs in a context. “Six days shall thou labour, and do all thy work: but the seventh day is the sabbath of the Lord thy God.” And it is not only in the context of the remainder of the commandment, but also in the context of the other commandments. “Thou shall have no other gods before me.” “I the Lord thy God am a jealous God.” “Thou shall not take the name of the Lord thy God in vain.” It is separation, therefore, to God, to the specific purpose of contemplation upon Him and specific occupation with His work in contrast with their own work. In this kind of distinction or sanctity the meaning of the fourth commandment resides. Abolish it, and the essence of the commandment is destroyed.
There is no purpose in contending for the moral obligation of the commandment unless this sanctity is recognized and preserved, for it is the core around which all else is formed and without which all else disintegrates. Just as there is an ineradicable distinction between the six days of creation and the day of rest by which they were followed, so it is here. And it is precisely with this reminder that the commandment itself ends, “For in six days the Lord made heaven and earth, the sea, and all that in them is, and rested the seventh day” wherefore the Lord blessed the sabbath day, and hallowed it.” Israel truly was a holy people; they were separated unto God Jehovah.
It might, then, be supposed that the sanctification of one day in seven was inconsistent with the totality of their devotion to God. Yet it is an inescapable fact that this kingdom of priests and holy nation was in the most direct way commanded to separate one day from the other six for a specific purpose. And unless our conception of devotion to God, and of time as it is related to Him, can embrace and appreciate this notion, together with the divine wisdom embodied in it, we can have no understanding of the fourth commandment.