Things always get a bit muddled when one mixes Judaism and Christianity. Transferring the Old Covenant obligations pertaining to the Sabbath to Sunday (which is not and never was the Sabbath) is but one example. Nowadays, Christians are all muddled in the head, thinking Sunday is a day of commanded rest and the day of which the Fourth Commandment speaks. The truth of the gospel, however, is that the seventh-day Sabbath was the sign of the old covenant between God and Israel. That covenant can be summed up simply as, "This do, and you shall live." Or, as Jesus put it, quoting the OT [paraphrased], "Man lives by keeping every word of God."
That, of course, was exactly how Jesus obtained eternal life: by keeping perfectly every commandment in the law. He, alone, was able to claim the promise of the old covenant. We cannot. Thankfully, Jesus has made a new covenant with us, as foretold by Ezekiel: "The justified ones shall obtain life through faith." Faith in Jesus Christ.
Thus, the keeping of Sabbaths and new moons and other holy days, while perhaps enjoyable and beneficial in certain respects, is not required of Christians, who are saved by faith and not by works of the law, which do not justify anyone.
Does this mean that all standards of morality and good behavior are repealed? Of course not. Standards of morality and good behavior did not come into being with the old covenant or the Ten Commandments, and the did not cease to exist when that covenant passed away. The standards of holiness have always existed, just as God has always existed. But observing a particular day of the week does not make one holy. It is a form, a symbol, a "shadow." It was part of an old covenant that is now obsolete.
Some suppose that God made the Sabbath at Creation. This is not accurate. God rested on the seventh day of Creation week and consecrated THAT single day. That was the first (and last) day in which mankind enjoyed perfect communion and rest with the Creator. At evening on that day, Adam and Eve profaned God's rest by taking of the forbidden fruit and God cursed them with hard labor--in childbirth and subsistence--and cast them out of the garden, out of his rest. On the eighth day--the first day of the new, sinful world--God ended his rest and commenced his labor of redeeming humanity and bringing them back into his rest.
There is no record that God ever commanded Adam and Eve or anyone else prior to Moses to keep a weekly Sabbath. Nor is there any record of anyone prior to Moses observing the weekly Sabbath. The weekly Sabbath created at Sinai was a shadow of the future, eternal, spiritual rest into which God desires to bring his people, through faith in Christ. The rest of God on the seventh day of Creation week was a singular, historical antecedent to the weekly Sabbath established at Sinai, much like the Pilgrim's thanksgiving celebration was a historical antecedent to the American national holiday that was not actually legally instituted until 1941.
So, in sum, the weekly seventh-day Sabbath is not a commanded day of rest "for all mankind," nor is it any longer God's holy day for Jews or Christians. It has passed away with the old covenant. The only path to God's rest is through the new covenant. We enter God's rest now through faith in Jesus Christ and reliance on his righteousness, not on our own deeds of obedience.
Anyone is free to observe a Saturday rest for any of various perfectly valid reasons, if he chooses, but he should be careful not to confuse his personal decision with God's commandment. "The righteous requirement of the law" is no longer efficacious; it is justification by faith in Christ that makes us righteous. If you start mingling in requirements to keep certain days and abstain from certain foods etc. then you are corrupting God's grace. You might as well try to do the whole law, instead of just picking and choosing a part here and there.
