The Sabbath is the seventh day of the Jewish week. Jewish days begin at sundown. The Sabbath begins at sundown on our Friday and ends at sundown on our Saturday. Sunday is the first day of a new week.
Israel were commanded to observe the Sabbath by doing no servile work. Observance was commanded in the Ten Commandments.
Exodus 20:8 Remember the sabbath day, to keep it holy. 9 Six days shalt thou labour, and do all thy work: 10 But the seventh day is the sabbath of the LORD thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates: 11 For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it.
The Ten Commandments written on stone tablets were part of the Old Covenant that God made with the nation of Israel at Sinai.
Israel failed to keep the Old Covenant and God promised a New Covenant.
Jeremiah 31:31 Behold, the days come, saith the LORD, that I will make a new covenant with the house of Israel, and with the house of Judah: 32 Not according to the covenant that I made with their fathers in the day that I took them by the hand to bring them out of the land of Egypt; which my covenant they brake, although I was an husband unto them, saith the LORD: 33 But this shall be the covenant that I will make with the house of Israel; After those days, saith the LORD, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people. 34 And they shall teach no more every man his neighbour, and every man his brother, saying, Know the LORD: for they shall all know me, from the least of them unto the greatest of them, saith the LORD: for I will forgive their iniquity, and I will remember their sin no more.
The writer to the Hebrews says:
Hebrews 8:8 For finding fault with them, he saith, Behold, the days come, saith the Lord, when I will make a new covenant with the house of Israel and with the house of Judah: 9 Not according to the covenant that I made with their fathers in the day when I took them by the hand to lead them out of the land of Egypt; because they continued not in my covenant, and I regarded them not, saith the Lord. 10 For this is the covenant that I will make with the house of Israel after those days, saith the Lord; I will put my laws into their mind, and write them in their hearts: and I will be to them a God, and they shall be to me a people: 11 And they shall not teach every man his neighbour, and every man his brother, saying, Know the Lord: for all shall know me, from the least to the greatest. 12 For I will be merciful to their unrighteousness, and their sins and their iniquities will I remember no more. 13 In that he saith, A new covenant, he hath made the first old. Now that which decayeth and waxeth old is ready to vanish away.
The New Covenant replaces the Old.
Jesus Christ is the mediator of the New Covenant ratified in his blood.
Hebrews 12:24 And to Jesus the mediator of the new covenant, and to the blood of sprinkling, that speaketh better things than that of Abel.
The commandments written on stone are replaced by the commandments of Christ.
John 15:14 Ye are my friends, if ye do whatsoever I command you.
In the commandments of Christ, 9 of the 10 commandments are repeated, but Sabbath observance is not.
The only observance commanded in the New Covenant is that of meeting often to share bread and wine in remembrance of Christ’s sacrifice.
1 Corinthians 11:23 For I have received of the Lord that which also I delivered unto you, That the Lord Jesus the same night in which he was betrayed took bread: 24 And when he had given thanks, he brake it, and said, Take, eat: this is my body, which is broken for you: this do in remembrance of me. 25 After the same manner also he took the cup, when he had supped, saying, This cup is the new testament in my blood: this do ye, as oft as ye drink it, in remembrance of me. 26 For as often as ye eat this bread, and drink this cup, ye do shew the Lord’s death till he come. 27 Wherefore whosoever shall eat this bread, and drink this cup of the Lord, unworthily, shall be guilty of the body and blood of the Lord. 28 But let a man examine himself, and so let him eat of that bread, and drink of that cup. 29 For he that eateth and drinketh unworthily, eateth and drinketh damnation (condemnation) to himself, not discerning the Lord’s body.
This can be done on any day.
The early church found it convenient to meet on the first day of the week and this became a tradition.
Meeting to share bread and wine is not to be confused with the Sabbath-keeping of the Old Covenant.
The difference between the Covenants is that the Old was designed for the governance of a nation, whereas the New is designed for the governance of an individual of any nation.
The carnal ordinances of the Old Covenant (such as dietary restrictions and observance of days) do not apply in the New.
Hebrews 9:10 Which stood only in meats and drinks, and divers washings, and carnal ordinances, imposed on them until the time of reformation.
Hosea 2:11 I will also cause all her mirth to cease, her feast days, her new moons, and her sabbaths, and all her solemn feasts.
Those who believe the gospel and are baptised into Christ are not commanded to observe the Sabbath.
The early Jewish believers could still observe the Sabbath, but gentile believers are not required to observe it.
Acts 21:18 And the day following Paul went in with us unto James; and all the elders were present. 19 And when he had saluted them, he declared particularly what things God had wrought among the Gentiles by his ministry. 20 And when they heard it, they glorified the Lord, and said unto him, Thou seest, brother, how many thousands of Jews there are which believe; and they are all zealous of the law: 21 And they are informed of thee, that thou teachest all the Jews which are among the Gentiles to forsake Moses, saying that they ought not to circumcise their children, neither to walk after the customs. 22 What is it therefore? the multitude must needs come together: for they will hear that thou art come. 23 Do therefore this that we say to thee: We have four men which have a vow on them; 24 Them take, and purify thyself with them, and be at charges with them, that they may shave their heads: and all may know that those things, whereof they were informed concerning thee, are nothing; but that thou thyself also walkest orderly, and keepest the law. 25 As touching the Gentiles which believe, we have written and concluded that they observe no such thing, save only that they keep themselves from things offered to idols, and from blood, and from strangled, and from fornication.
Paul writes to the Romans:
Romans 14:5 One man esteemeth one day above another: another esteemeth every day alike. Let every man be fully persuaded in his own mind. 6 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.
And to the Colossians:
Colossians 2:16 Let no man therefore judge you in meat, or in drink, or in respect of an holyday, or of the new moon, or of the sabbath days: 17 Which are a shadow of things to come; but the body is of Christ.
And to the Galatians:
Galatians 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? 10 Ye observe days, and months, and times, and years. 11 I am afraid of you, lest I have bestowed upon you labour in vain.
Galatians 4:21 Tell me, ye that desire to be under the law, do ye not hear the law? 22 For it is written, that Abraham had two sons, the one by a bondmaid, the other by a freewoman. 23 But he who was of the bondwoman was born after the flesh; but he of the freewoman was by promise. 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. 25 For this Agar is mount Sinai in Arabia, and answereth to Jerusalem which now is, and is in bondage with her children. 26 But Jerusalem which is above is free, which is the mother of us all. 27 For it is written, Rejoice, thou barren that bearest not; break forth and cry, thou that travailest not: for the desolate hath many more children than she which hath an husband. 28 Now we, brethren, as Isaac was, are the children of promise. 29 But as then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now. 30 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. 31 So then, brethren, we are not children of the bondwoman, but of the free.
Galatians 5:1 Stand fast therefore in the liberty wherewith Christ hath made us free, and be not entangled again with the yoke of bondage.
Sabbath observance was part of the ‘bondage’ of the Law.
The penalty for no-observance was death.
Christ has freed us from the Law given through Moses.
In the kingdom of God, Sabbath observance will be reinstated as part of national governance.
Ezekiel 45:17 And it shall be the prince’s part to give burnt offerings, and meat offerings, and drink offerings, in the feasts, and in the new moons, and in the sabbaths, in all solemnities of the house of Israel: he shall prepare the sin offering, and the meat offering, and the burnt offering, and the peace offerings, to make reconciliation for the house of Israel.
Ezekiel 46:3 Likewise the people of the land shall worship at the door of this gate before the LORD in the sabbaths and in the new moons.
In conclusion: Sunday is not the Sabbath and Sabbath observance is not required of those who believe the gospel and are baptised into Jesus Christ.
May I suggest that you read:
Basic Bible Teaching - The Sabbath: https://www.thisisyourbible.com/searchresult/36
Sunday and the Sabbath: https://www.thisisyourbible.com/library/show/176
I hope you find this helpful.