Christians are not Jews laboring under the laws of Judaism in the old covenant that God had with the nation of Israel. Rather, Christians (genuine Christians anyways) are new creatures in Christ under the new Covenant God established through Jesus Christ.
We do not observe the literal sabbath day under the old covenant for our salvation, because Christ is our Sabbath. We rest in him. In Colossians 2:16-17, the Apostle Paul forbids the observance of any legal sabbath day on the basis of the fact that in Christ believers are entirely free from the law (Rom. 6:14, 15; 7:4; 10:4). The word translated "rest" in Hebrews 4:9 might be translated "sabbath" or "keeping of a sabbath." The sabbath rest spoken of in the chapter is the blessed rest of faith in the Lord Jesus Christ (vv. 3, 4, 9, 10) and the consummate end of that in eternal glory (v. 11).
Those who would impose upon us the legal observance of sabbath days and bring us back in bondage to the law are falling into the error of the Judaizers spoken against by the Apostles. Many preach up sabbath keeping as a rule of life and try to impose upon God's saints rules and regulations for sabbath keeping. But it is all a vain and hypocritical show of the flesh (Gal. 6:12-13). Not one of them actually keeps the sabbath day; like the Jews were supposed to under the old covenant. Their pretended reverence for the law of God, when closely examined, reveals a total disregard for God's law. They sift through the commandments, pick out what they like, ignore the rest, and then justify it with cultic theologies.
We do not and must not observe a literal sabbath day for the same reasons that we dare not observe the Jewish passover or any other ceremonies of the law. TO DO SO WOULD BE A SYMBOLIC DENIAL OF THE FINISHED WORK OF CHRIST AS OUR REDEEMER, A MIXING OF WORKS AND GRACE IN THE AFFAIR OF SALVATION, AND A DECLARATION THAT CHRIST ALONE IS NOT ENOUGH TO SATISFY GOD AND MAKE ALL WHO TRUST HIM FOREVER ACCEPTED WITH GOD.