The first two chapters of the book of Genesis teach seventh day Sabbath observance. The fourth of the ten commandments in Exodus chapter 20 and Deuteronomy chapter 5 is almost unnecessary. Of course, the fourth commandment is important because it makes it clear beyond any doubt to an honest and open mind that Sabbath observance is definitely commanded. But even without the fourth commandment, I think a mind submitted to seek and do God's will can discern from the first two chapters of Genesis that Sabbath observance is God's will.
You have to think about what Genesis 1:1 thru Genesis 2:3 says and means, and the consequences of what it says. There is more than meets the eye in a first quick reading. There are lessons we can draw if we meditate on this portion of God's word.
"Oh, how I love Your law! It is my meditation all the day" (Psalm 119:97). "Make me understand the way of Your precepts; So shall I meditate on Your wonderful works" (Psalm 119:27). "I remember the days of old; I meditate on all Your works; I muse on the work of Your hands" (Psalm 143:5).
The first verse in Genesis shows that God is Creator. "In the beginning God created the heavens and the earth" (Genesis 1:1).
Then, verses 3 through 25 show how God, in His work of creation, took six days to renew and restore the surface of the earth. He didn't just do it instantly, in a moment, though we know from other parts of the Bible that He could have if He wanted to. But He took time to do it. He did the work of creation in stages over six days. It was His choice to do it that way.
It seems reasonable to ask, why did God take six days to do this in stages rather than instantly, all at once?
Verses 26-27 show that God made man and therefore has authority over man. They also show that God made man to be in His own likeness. "Then God said, 'Let Us make man in Our image, according to Our likeness; let them have dominion over the fish of the sea, over the birds of the air, and over the cattle, over all the earth and over every creeping thing that creeps on the earth.' So God created man in His own image; in the image of God He created him; male and female He created them" (Genesis 1:26-27). This is important, because once we know it is God's will that we be in His "image", we can see a purpose for God to do the work of recreating the surface of the earth in steps, over six days. He did this to set an example for us, the example that we should work. As it takes man time to accomplish things through work, so God took time to do His work. This lesson, that man should spend time working productively, is reinforced by Genesis 2:15: "Then the Lord God took the man and put him in the garden of Eden to tend and keep it."
In other words, we see that it is God's will that we be in His image, and we can see that God set an example for us, the example of spending time working productively, by taking time to recreate the surface of the earth, to show us, by example, that we should spend our time working and not living a life of idleness.
Then, God set the example of resting on the seventh day. "And on the seventh day God ended His work which He had done, and He rested on the seventh day from all His work which He had done" (Genesis 2:2).
Why did God inspire the Genesis account to say that He rested on the seventh day? God does not get tired. Also, if He finished His work of creation in six days, why say that He rested on the seventh day specifically? Did He do any creating on the eighth day, the ninth day, the tenth day, or the eleventh day? Didn't He "rest" on those days from the work of renewing the surface of the earth as much as on the seventh day? But it is the seventh day that God says He rested on.
Again, a discerning mind that is seeking to know God's will so he can do it will ask, what is God telling me here? Is God setting an example that I should follow, of working the first six days of the week and resting on the seventh day? Should I follow God's example because He made me to be in His image and likeness? And if God did not give me this as an example, why did He take six days to renew the face of the earth and then tell me specifically that He "rested" on the seventh day?
The answer becomes more clear in the next verse. "Then God blessed the seventh day and sanctified it, because in it He rested from all His work which God had created and made" (Genesis 2:3). God clearly tells us that He sanctified, set apart, the seventh day. It is to be different from other days. And He puts a special blessing on that day.
It would make no sense for God to tell us He blessed that first seventh day after creation and sanctified and set it apart if it was only that one day and not the seventh day in every seven-day weekly cycle that is sanctified and blessed. It's not rocket science to discern from this account in Genesis only that we should honor God and follow His example, to rest the seventh day from our work as God did from His work, to work six days as God worked, but to keep the seventh day special, to rest on that day, to recognize God's special blessing on that day. He made us in His image. He set this example and inspired the writing of the account of this for a reason, so we can do likewise.
The fourth commandment does not establish the weekly Sabbath, but reminds us of it and makes it clear beyond doubt that its observance is not optional. That is why the fourth commandment tells us, remember the Sabbath and keep it holy (Exodus 20:8-11).
Here are links to related chapters or sections in Preaching the Gospel:
The Weekly Sabbath Day, Chapter 2