Genesis 2:2 And on the seventh day, God completed His work, which He had done, and He rested on the seventh day from all His work which He had done.
The Bible doesn't say that God legally obligated Adam to rest on the seventh day. Before the fall, Adam's work, to take dominion over the Earth and subdue it for the Lord, was acceptable before God. Why would God command him to rest from such a good work? It should also be noted that the Hebrew word for 'work' is interchangeble with 'worship'.
Exodus 31:15 Six days work may be done, but on the seventh day there is a sabbath of complete rest, holy to Yahweh; whoever does any work on the sabbath day shall surely be put to death.
God prohibits the Hebrews from working on the Sabbath. Why is this? I suggest it is because their works are evil and their worship is unacceptable. Like Adam, they are transgressors of the covenant. They are legally obligated to cease from their evil works on the Sabbath. Not that the works that the Law commands are evil, but that the Hebrews in their evilness are unable to follow the works of the Law, and so they must rest from doing them. Hebrews says that they did not pursue God's righteousness through faith, but as if it were by their own works. Immediately following this command is the account of the golden calf, to further drive home the point. The Old Testament also later says that they profaned the Sabbath, not being able to stop doing their evil works even on the Sabbath.
John 5:16 And for this reason the Jews were persecuting Jesus, because He was doing these things on the Sabbath. 17But He answered them, “My Father is working until now, and I Myself am working.” 18 For this reason therefore the Jews were seeking all the more to kill Him, because He not only was breaking the Sabbath, but also was calling God His own Father, making Himself equal with God.
How is it that Jesus can work on the Sabbath and not be guilty of breaking the Law? Because, like Adam in the beginning, His works are acceptable before God. All of His works are acts of true worship toward God. According to His own words, He works on the Sabbath. He has no sin that He is required by the Law to cease from doing. So where does that leave us who are in Christ by faith?
Galatians 5:22 But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, 23 gentleness, self-control. Against such things there is no law. 24 Now those who belong to Christ Jesus crucified the flesh with its passions and desires.
There is no Law against our worship, against our works. Christian worship is not brought on us by means of legal obligation but by God's free grace. Whether we observe the day or not, we do it unto God who makes us stand.
Ephesians 2:8 For by grace you have been saved through faith, and this not of yourselves, it is the gift of God; 9not of works, so that no one may boast. 10For we are His workmanship, created in Christ Jesus for good works, which God prepared beforehand so that we would walk in them.
We did not work our way into this salvation. God worked to give it to us when we rested in Him through faith. And having been justified, our works are now acceptable worship to Him. So let us continue to work in the worship He has predestined us to walk in. All of the world's religions say that we must worship in order to get saved, but God says that we have been saved in order to worship.