Without going into too great detail of a subject which is at the heart of the Gospel and, indeed, all of God's revelation to us, these two verses do not point up a discrepancy or contradiction but, instead, illuminate a difference between two covenants; one of law and the other of promise.
If there is considerable debate within Christendom concerning the role of the law (and there is) then the atheist is at a severe disadvantage in that they cannot ascribe promise keeping power to a deity they do not acknowledge.
The command (s) of the law, of which Sabbath keeping is one, given by God to Israel through Moses were not given to produce righteousness but, instead, were given to delineate and condemn unrighteousness:
Wherefore then serveth the law? It was added because of transgressions, till the seed should come to whom the promise was made; and it was ordained by angels in the hand of a mediator. Now a mediator is not a mediator of one, but God is one. Is the law then against the promises of God? God forbid: for if there had been a law given which could have given life, verily righteousness should have been by the law. - Galatians 3:19-21
Just as the law "Thou shalt not commit adultery..." was spoken to a humanity that is prone to it and, just as obeying this law outwardly does not remove the inner proclivity:
Ye have heard that it was said by them of old time, Thou shalt not commit adultery:
But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart. - Matthew 5:27-28
so outward obedience to the Sabbath law does not remove the inner disposition to become unmindful of the Creator.
The Law, says the apostle Paul, was added because of transgressions [until the promised seed should come]. Paul (in 2 Corinthians 3) also refers to the law as a ministry of death and as a ministry of condemnation. He does so, not to point out a flaw in the law but to point out the flaw in man which the law was intended to restrain until the flaw in man is remediated. This is the Old Covenant. That schoolmaster or guardian kept us until the remediation occurred:
But the scripture hath concluded all under sin, that the promise by faith of Jesus Christ might be given to them that believe. But before faith came, we were kept under the law, shut up unto the faith which should afterwards be revealed. Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith. But after that faith is come, we are no longer under a schoolmaster. For ye are all the children of God by faith in Christ Jesus. - Galatians 3:22-26
The New Covenant is that remediation. Jesus has represented humanity in the perfect keeping of the Old Covenant in that he positively never disobeyed either in word, deed, or intent (no sin was found in Him) and also in that he bore humanity's penalty for disobedience in Himself:
Who his own self bare our sins in his own body on the tree, that we, being dead to sins, should live unto righteousness: by whose stripes ye were healed. - 1 Peter 2:24
Once one comes to Christ there is no longer any need for the guardian/law and this is not because adultery is now permitted but because the Christian now has been given a new spirit ... the guardian is no longer external (law) but internal. It is actually the Spirit of the Living God, the Spirit of Christ, who lives within and teaches and enables the believer to keep the righteous requirement of the law:
There is therefore now no condemnation to them which are in Christ Jesus, who walk not after the flesh, but after the Spirit. For the law of the Spirit of life in Christ Jesus hath made me free from the law of sin and death. For what the law could not do, in that it was weak through the flesh, God sending his own Son in the likeness of sinful flesh, and for sin, condemned sin in the flesh: That the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit. - Romans 8:1-4
This is why Paul is able to say "One man esteemeth one day above another: another esteemeth every day alike. Let every man be fully persuaded in his own mind." and that is clarified in the following verses:
He that regardeth the day, regardeth it unto the Lord; and he that
regardeth not the day, to the Lord he doth not regard it. He that
eateth, eateth to the Lord, for he giveth God thanks; and he that
eateth not, to the Lord he eateth not, and giveth God thanks. For none
of us liveth to himself, and no man dieth to himself. For whether we
live, we live unto the Lord; and whether we die, we die unto the Lord:
whether we live therefore, or die, we are the Lord's. For to this end
Christ both died, and rose, and revived, that he might be Lord both of
the dead and living. - Romans 14:6-9
As can be seen, under the new covenant the believer who keeps one particular Sabbath and the believer who considers every day as the Lord's have one thing in common ... they regard it unto the Lord. This is what the law is schoolmaster of and what Jesus has brought to pass:
And whatsoever ye do in word or deed, do all in the name of the Lord Jesus, giving thanks to God and the Father by him. - Colossians 3:17