Fear God, and keep his commandments: for this is the whole duty of man., page-7

As we know the Fear of God is the beginning of knowledge (Proverbs 1:7) and the Ten Commandments were known and obeyed by faithful people before the law was given at Sinai. Later incorporated into the new covenant, they constitute the basic moral code for humanity (as further explained by Jesus) and are obeyed to demonstrate the believer’s love for God and his fellow man.
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CEO, it is not true that the 10 Commandments were known before the law was given at Sinai. Faithful men and women were guided by their conscience that was shaped by their love for God, but it did not always lead to them making the right choices.
For example, polygamy was widely practiced by the forefathers of the only nation to be given the Law.[Psalm 147:19,20]. Jesus made it clear this was not by God's direction. Jesus showed that God set the standard for marriage when He told the first human pair: “A man will leave his father and his mother and will stick to his wife, and the two will be one flesh.” (Matthew 19:5) Marriage was to be a lasting bond between two persons.
Though God did not originate or encourage polygamy, he provided laws to regulate it. When the Christian congregation was formed, God’s Word clearly prohibited polygamy.
(1 Timothy 3:2) The overseer should therefore be irreprehensible, a husband of one wife, moderate in habits, sound in mind, orderly, hospitable, qualified to teach,

It is similar with the Sabbath law. God used the Sabbath as a temporary arrangement to teach people that they should regularly take time to meditate on spiritual matters. It pointed forward to the time when mankind would find lasting rest from physical and spiritual afflictions.

(Hebrews 4:10) For the man who has entered into God’s rest has also rested from his own works, just as God did from his own.

Christians are not commanded to reserve any one day to reflect on or observe God's requirements.