God does not specifically identify Himself with any other day of the week, and He commands His people to meet with Him on no other day. These truths are so strong that God includes the Sabbath in the ten foundational laws governing morality. How much plainer can it get? In addition, the apostle Paul says this body of laws is spiritual (Romans 7:14). This has universal and eternal ramifications, further enhanced by the fact that Jesus kept it (and we are to follow His example, I John 2:4-6), as did the apostles.
God created the Sabbath because it enhances and protects our relationship with Him. It provides a witness to God, to ourselves and to the world. It keeps us in a proper frame of mind and furnishes us with the right knowledge of our part of the pilgrimage to God's Kingdom.
We live in a grubby, grasping, materially oriented world, where a built-in bias exists toward materialism and the exercise of carnality. If we follow it, we can find it is not hard at all to avoid spiritual things. But keeping the Sabbath almost forces us to think about God, the spiritual side of life and His creation. It presents us with opportunities to consider the WHYS of life, to get ourselves correctly oriented to use our time properly the other six days. Keeping the Sabbath correctly is the kernel, the nucleus, from which grows appropriate worship (our response to God).
Existentialist philosophers tell us that life is absurd. They say that all life is but a prelude to death. The Sabbath celebrates just the opposite! It reminds us that God's creative process is continuing. God is creating us in His image so that physical life is not absurd but a prelude to life on an infinitely higher, spiritual level. As we grow more like Him, we become more sanctified from this world. In experiencing, refreshing, and elevating the mind in the realm of the spirit, we get a foretaste of what is to come.