***“Heb 4:9 There remaineth therefore a rest(sabbath) to the people of God.”***
This rest is found by being IN CHRIST, not in observing any day. Being IN CHRIST means we have CEASED from our own labors of trying to come up to the impossible standards of the LAW. No sacrifices. No law of Moses, No circumcision. We are at rest constantly IN CHRIST.
Rom 7:7 ¶ What shall we say then? [Is] the law sin? God forbid. Nay, I had not known sin, but by the law: for I had not known lust, except the law had said, Thou shalt not covet.
Gal 2:21 I do not frustrate the grace of God: for if righteousness [come] by the law, then Christ is dead in vain.
Gal 3:1 ¶ O foolish Galatians, who hath bewitched you, that ye should not obey the truth, before whose eyes Jesus Christ hath been evidently set forth, crucified among you?
Gal 3:2 This only would I learn of you, Received ye the Spirit by the works of the law, or by the hearing of faith?
Gal 3:3 Are ye so foolish? having begun in the Spirit, are ye now made perfect by the flesh?
Gal 4:9 But now, after that ye have known God, or rather are known of God, how turn ye again to the weak and beggarly elements, whereunto ye desire again to be in bondage?
Gal 4:10 Ye observe days, and months, and times, and years.
Gal 4:11 I am afraid of you, lest I have bestowed upon you labour in vain.
Gal 4:21 ¶ Tell me, ye that desire to be under the law, do ye not hear the law?
Gal 4:22 For it is written, that Abraham had two sons, the one by a bondmaid, the other by a freewoman.
Gal 4:23 But he [who was] of the bondwoman was born after the flesh; but he of the freewoman [was] by promise.
Gal 4:24 Which things are an allegory: for these are the two covenants; the one from the mount Sinai, which gendereth to bondage, which is Agar.
Gal 4:25 For this Agar is mount Sinai in Arabia, and answereth to Jerusalem which now is, and is in bondage with her children.
The word used is sabbath and Jesus is the lord of the sabbath. The problem is you can not prove that Sunday replaces Saturday for the sabbath day using the Bible. The seventh day(Saturday)was sanctified before Moses or Abraham Genesis 2:3 that is before the Hebrews. Paul had Timothy circumcised Acts 16:3 not because the law of Moses but because of Genesis 17:10. As many times the Lord was accused of breaking the sabbath I would expect that had the Church adopted Sunday at an early date then it would have been mentioned as an issue in Acts or the epistles on account that Gentiles were a Minority and Jews would have argued about it.