Then to keep the whole law you must be circumcised, but Paul, by the Holy Spirit said later that if you are circumcised, that what Christ did on the cross will not do you any good.
Behold, I Paul say unto you, that if ye be circumcised, Christ shall profit you nothing. For I testify again to every man that is circumcised, that he is a debtor to do the whole law. Christ is become of no effect unto you, whosoever of you are justified by the law; ye are fallen from grace.
Gal 5:2-4
Read Hebrews 4 about the New Testament Sabbath
For he spake in a certain place of the seventh day on this wise, And God did rest the seventh day from all his works. And in this place again, If they shall enter into my rest. Seeing therefore it remaineth that some must enter therein, and they to whom it was first preached entered not in because of unbelief: Again, he limiteth a certain day, saying in David, To day, after so long a time; as it is said, To day if ye will hear his voice, harden not your hearts. For if Jesus had given them rest, then would he not afterward have spoken of another day. There remaineth therefore a rest to the people of God. For he that is entered into his rest, he also hath ceased from his own works, as God did from his. Let us labour therefore to enter into that rest, lest any man fall after the same example of unbelief.
Heb 4:4-11
The New Testament Sabbath is to cease from our old works which is our old sinful lifestyle. We are to rest in obedient faithful love to our Lord and Savior Jesus Christ. The New Testament Sabbath is far more serious than the Old Testament Sabbath and it has nothing to do with a specific day. As someone already quoted Romans 14:5, if someone wants to esteem one day over another, that's okay, but if someone else wants to treat all days alike, that's okay too. Romans 14 is about not condemning other people for observing or not observing certain days, eating or not eating meat and other such things weaker and stronger Christians might clash on. Yet we have entire denominations, web sites, various "ministries" and such like, completely devoted to arguing about them. According to Paul, speaking by the Holy Spirit, if you seek to justify yourself on the basis that you keep any part of the law, you are obligated to keep every jot and tittle thereof. That means you are obligated to every holy day, every sacrifice, every food restriction, etc. I believe I heard there are something close to 700 precepts of the Mosaic law. The early church argued about this at a special counsel and they gave very specific instructions as to the fact that Gentile Christians are not obligated to the whole Mosaic law and the few precepts they did say were important did not include the legal seventh day Sabbath.
For it seemed good to the Holy Ghost, and to us, to lay upon you no greater burden than these necessary things; That ye abstain from meats offered to idols, and from blood, and from things strangled, and from fornication: from which if ye keep yourselves, ye shall do well. Fare ye well.
Act 15:28-29
How did they convince the assembly to agree to this? With a very sobering plea from Peter:
Now therefore why tempt ye God, to put a yoke upon the neck of the disciples, which neither our fathers nor we were able to bear? But we believe that through the grace of the Lord Jesus Christ we shall be saved, even as they. Then all the multitude kept silence, and gave audience to Barnabas and Paul, declaring what miracles and wonders God had wrought among the Gentiles by them.
Act 15:10-12
Why indeed must so many ministries be devoted to putting this yoke on believers today? They may say they are only promoting the Sabbath for pre-law reasons and such like, but even animal sacrifice was pre-law and that if you put yourself under the obligation of even one small commandment for the law's sake, you are obligated to keep every commandment. The Bible is clear how God feels about the Mosaic Law for the New Testament believer. Read Galatians as a whole. It deals directly with people trying to take us back to the Mosaic law and it is very clear that law keepers and grace receivers will not be heirs together but the law keepers will be cast out.
Now we, brethren, as Isaac was, are the children of promise. But as then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now. Nevertheless what saith the scripture? Cast out the bondwoman and her son: for the son of the bondwoman shall not be heir with the son of the freewoman. So then, brethren, we are not children of the bondwoman, but of the free.
Gal 4:28-31
Again this is not a license to sin, just a change of the law. Hebrews 4 makes the New Testament Sabbath a rest from sin and therefore far more serious than simply a day off work.
For the priesthood being changed, there is made of necessity a change also of the law.
Heb 7:12
May I say with Peter, Why tempt ye God to put a yoke on believers today that no believer in history has ever been able to bear except Jesus?