Question -- But why do not we Christians under the New, keep the Sabbath on the same seventh day on which it was kept under the Old Testament?
I answer -- Because our Lord Jesus, who is the Lord of the Sabbath (Matt. xii.8), and whom the law itself commands us to hear (Deut. xviii.18, 19), did alter it from that seventh day to this first day of the week on which we keep the Sabbath. For the holy evangelist notes, that our Lord came into the midst of the holy assembly on the two first days of the two weeks immediately following his resurrection, and then blessed the church, breathed on the apostles the Holy Ghost, and gave them the ministerial keys, and power of binding and remitting sins (John xx.22, 23.) And so it is most probable he did in a solemn manner every first day of the week, during the forty days he continued on earth between his resurrection and ascension (for the fiftieth day after, being the first day of the week, the apostles were assembled;) during which time he gave commandments unto the apostles, and "spake unto them those things which appertain to the kingdom of God," (Acts i.2, 3),  that is, instructed them how they should, throughout the churches which were to be converted, change the Sabbath to the Lord's day; the bodily sacrifices of beasts, to the spiritual sacrifices of praise, prayer, and contrite hearts; the Levitical priesthood of the law, to the Christian ministry of the gospel (Heb. vii.11, 12;) the Jewish temples and synagogues, to churches and oratories; the old sacraments of circumcision and passover, to baptism and the Lord's Supper, &c. as may appear by the like phrase (Acts xix.8; xxviii.23; Col. iv.11), put for the whole sum of Paul's doctrine, by which were wrought all these changes, where it took effect. So that as Christ was forty days instructing Moses in Sinai, what he should teach, and how he should rule the church under the law; so he continued forty days teaching his disciples in Sion, what they should preach, and how they should govern the church under the gospel (Eph. iv.8, 11, 12.) And seeing it is manifest, that within those forty days Christ appointed what ministers should teach, and how they should govern his church to the world's end, it is not to be doubted but that within those forty days he likewise ordained on what day they should keep their Sabbath, and ordinarily do the works of their ministry; especially seeing that under the Old Testament, God shewed himself as careful, both by his moral and ceremonial law, to prescribe the time, as well as the matter of his worship. Neither is it a thing to be omitted, that the Lord, who hath times and seasons in his own power (Acts i.7), appointed this first day of the week to be the very day in which he sent down from heaven the Holy Ghost upon the apostles (Acts ii.1, &c.), so that upon that day they first began, and ever after continued the public exercising of their ministry, in the preaching of the word (Acts ii.1, 4), the administration of the sacraments (Acts ii.38, 41, 42), and the loosing of the sins of penitent sinners (Acts ii.38.) Upon these, and the like grounds, Athanasius plainly affirms that the Sabbath-day was changed by the Lord himself. 
As, therefore, our communion is termed the Lord's Supper, because it was instituted of the Lord, for the remembrance of his death; so the Christian Sabbath is called the Lord's day (Rev. i.10), because it was ordained of the Lord, for the memorial of his resurrection.  And as the name of the Lord (1 Cor. xi.10) honours the one, so does it the other; and as the Lord of the Sabbath, by his royal prerogative, and transcendent authority, could, so he had also reason to change the holy Sabbath from the seventh day to this, on which we keep it; for as concerning the seventh day, which followed the six days in which God finished the creation, there was no such precise institution, or necessity of sanctifying it perpetually, but such as by the same authority, or upon greater reason and occasion, it might very well be changed and altered to some other seventh day; for the commandment does not say, Remember to keep holy the seventh day next following the sixth day of the creation, or this or that seventh day; but indefinitely, Remember that thou keep holy a seventh day.  And to speak properly, as we take a day for the distinction of time, called either a day natural, consisting of twenty-four hours, or a day artificial, consisting of twelve hours, from sun rising to sun setting; and withal consider the sun standing still at noon, in Joshua's time (Josh. x.12, 43) the space of a whole day; and the sun going back ten degrees (2 Kings xx.11), viz. five hours, almost half an artificial day, in Ezechiah's time; the Jews themselves could not keep their Sabbath upon that precise and just distinction of time, called at the first the seventh day from the creation.
Add to this, that in respect of the diversity of meridians, and the unequal rising and setting of the sun, every day varies in some places a quarter, in some half, in others a whole day. Therefore the Jewish seventh day cannot precisely be kept at the same instant of time every where in the world.
Now our Lord Jesus having authority as Lord over the Sabbath (Matt. xii.8), had likewise now far greater reason and occasion to translate the Sabbath from the Jewish seventh day to the first day on which Christians keep the Sabbath.
1. Because that by his resurrection from the dead, there is wrought a new spiritual creation of the world, without which all the sons of Adam had been turned to everlasting destruction, and all the works of the first creation had ministered no consolation to us (Isa. lxv.17, &c.; lxvi.22; Psal. xc.2.)
2. And in respect of this new spiritual creation, the Scripture saith, "That old things are passed away, and all things are become new," (2 Cor. v.17) -- new creatures (Gal. vi.15), new people (1 Pet. ii.10), new men (Eph. iv.24), new knowledge (Col. iii.10), new testament (Matt. xxvi.28), new commandment (John xii.13), new names (Rev. ii.17), new way (Heb. x.10), new song (Rev. iii.9), new garment (Luke v.36, 37), new wine, new vessels (Rev. xxi.2), new Jerusalem (2 Pet. iii.13), new heaven, and a new earth (Isa. lxvi.22) And therefore of necessity there must be, instead of the old, a new Sabbath-day (Heb. iv.9), to honour and praise our Redeemer, and to meditate upon the work of our redemption, and to shew the new change of the Old Testament.
3. Because that on this day Christ rested from all the sufferings of his passion, and finished the glorious work of our redemption. If, therefore, the finishing of the work of the first creation, by which God mightily manifested himself to his creatures, deserved a sabbath to solemnize the memorial of so great a work, to the honour of the worker, and therefore calls it "mine holy day," (Isa. lviii.13;) much more does the new creation of the world, effected by the resurrection of Christ, whereby he mightily declared himself to be the Son of God (Rom. i.4), deserve a sabbath, for the perpetual commemoration of it, to the honour of Christ, and therefore worthily called the Lord's day (Rev. i.10.) For, as the deliverance out of the captivity of Babylon, being greater, took away the name from the deliverance out of the bondage of Egypt (Jer. xxiii.7, 8;) so the day on which Christ finished the redemption of the world did more justly deserve to have the Sabbath kept on it, than on that day on which God ceased from creating the world. As, therefore, in the creation, the first day wherein it was finished, was consecrated for a sabbath; so in the time of redemption, the first day wherein it was perfected must be dedicated to a holy rest; but still a seventh day kept according to God's moral commandment. The Jews kept the last day of the week, beginning their Sabbath with the night (Gen. ii.2; Lev. xxiii.32; Neh. xiii.19), when God rested; but Christians honour the Lord better, on the first day of the week (Matt. xxviii.1), beginning the Sabbath with the day when the Lord arose (Acts xx.7, 11.) They kept their Sabbath in remembrance, of the world's creation; but Christians celebrate it in memorial of the world's redemption; yea, the Lord's day being the first of the creation and redemption, puts us in mind, both of the making of the old, and redeeming of the new world.
As, therefore, under the Old Testament, God, by the glory consisting of seven lamps, seven branches, &c. (Exod. xxv.31), put them in remembrance of the creation, light, and Sabbath's rest; so, under the New Testament, Christ, the true light of the world, appears in the midst of the seven lamps, and seven golden candlesticks (Rev. i.13), to put us in mind to honour our Redeemer in the light of the gospel of the Lord's seventh day of rest. And seeing the redemption, both for might and mercy, so far exceeds the creation, it stood with great reason that the greater work should carry the honour of the day. Neither does the honourable title of the Lord's day diminish the glory of the Sabbath; but rather, being added, augments the dignity of it; as the name Israel, added to Jacob, made the patriarch the more renowned (Gen. xxxii.28.)
The reason taken from the example of God's resting from the work of the creation of the world continued in force till the Son of God ceased from the work of the redemption of the world, and then the former gave place to the latter.
4. Because it was foretold in the Old Testament that the Sabbath should be kept under the New Testament on the first day of the week.
For, first, in the 110th Psalm, which is a prophecy of Christ and his kingdom, it is plainly foretold that there should be a solemn day of assembling, wherein all Christ's people should willingly come together in the beauty of holiness (Psal. cx.3;) insomuch that no rain (of peace) shall be upon those families that in that feast will not go up to Jerusalem (the church) to worship the King, the Lord of Hosts (Zech. xiv.17.) Now on what day this holy feast and assembly should be kept, David shews plainly in Psalm cxviii., which was a prophecy of Christ, as appears, Matt. xxi.42; Acts iv.11; Eph. ii.20, as also by the consent of all the Jews, as Jerome witnesses. For, shewing how Christ, by his ignominious death, should be as a stone rejected of the builders, or chief rulers of Judea, and yet, by his glorious resurrection, should become the chief stone of the corner, he wishes the whole church to keep holy that day upon which Christ should effect this wonderful work, saying, "This is the day which the Lord hath made, let us rejoice and be glad in it." (Psal. cxviii.24.) And seeing that upon this day that which Peter says of Christ appears to be true, that "God made him both Lord and Christ." (Acts ii.36.) Therefore the whole church under the New Testament must celebrate the day of Christ's resurrection. Rabbi Bachay also saw, by the fall of Adam on the sixth day, that on the same day Messias should finish the work of man's redemption; and, alluding to the speech of Boaz to Ruth, "sleep unto the morning," that Messias should rest in his grave all their Sabbath-day. And he gathers from that speech (Gen. i.) on the first day, "Let there be light," that the Messias should rise on the first day of the week from death to life, and cause the spiritual light of the gospel to enlighten the world, that lay in the shadow of darkness and death. The Hebrew author of the book called Sedar Olam Rabbi (cap.7) records many memorable things which were done upon the first day of the week, as so many types that the chief worship of God should, under the New Testament, be celebrated upon this day: As, that on this day the cloud of God's majesty first sate upon his people; Aaron and his children first executed their priesthood; God first solemnly blessed his people; the princes of his people first offered publicly unto God; the first day wherein fire descended from heaven; the first day of the world, of the year, of the month, of the week, &c.; all shadowing that it should be the first and chief holy day of the New Testament. St. Augustine proves, by divers places and reasons out of the holy Scripture, that the fathers, and all the holy prophets under the Old Testament, did foresee and know that our Lord's day was shadowed by their eighth day of circumcision, and that the Sabbath should be changed from the seventh day to the eighth, or first day of the week. And Junius, out of Cyprian, says, that circumcision was commanded on the eighth day, as a sacrament of the eighth day, when Christ should arise from the dead. The council Foro Juliense affirms, that Isaiah prophesied of the keeping of the Sabbath upon the first day of the week. If this mystery was so clearly seen by the Fathers under the shadows of the Old Testament, surely the god of this world has deeply blinded their minds who cannot see the truth of it under the shining light of the gospel (2 Cor. iv.4.) Therefore this change of the Sabbath-day under the New, was nothing but a fulfilling of that which was prefigured and fore-prophesied under the Old Testament.
5. According to their Lord's mind and commandment, and the direction of the Holy Ghost, which always assisted them in their ministerial office, the apostles, in all the Christian churches which they planted, ordained that the Christians should keep the holy Sabbath upon that seventh day, which is the first day of the week: "Concerning the gathering for the saints, as I have ordained in the churches of Galatia, so do ye also. Every first day of the week," &c. (1 Cor. xvi.1, 2.) "When ye come together in the church (being the Lord's day) to eat the Lord's Supper,  to remember and shew the Lord's death till he come," &c. (1 Cor. xi.20, 25, 26.) In which words note --
(1.) That the apostle ordained this day to be kept holy: therefore a divine institution.
(2.) That the day is named the first day of the week: therefore not the Jewish seventh, or any other.
(3.) Every first day of the week, which shews a perpetuity.
(4.) That it was ordained in the churches of Galatia, as well as of Corinth, and he settled one uniform order in all the churches of the saints: therefore it was universal.
(5.) That the exercises of this day were collections for the poor (which appears by Acts ii.42, and Justin Martyr's testimony, Apolog. ii.  ), which were gathered in the holy assembly after prayer, preaching of the word, and administration of the sacraments: therefore it was spiritual.
(6.) That he will have the collection, though necessary, removed against his coming; lest it should hinder his preaching; but not their holy meeting on the Lord's day, for it was the time ordained for the public worship of the Lord, which argues a necessity.
And in the same Epistle St. Paul protests, that he delivered them none other ordinance or doctrine but what he hath received of the Lord (1 Cor. xi.23.) Insomuch that he charges them, that "if any man think himself to be a prophet, or spiritual, let him acknowledge that the things that I write unto you are the commandments of the Lord." (1 Cor. xiv.37.) But he wrote unto them, and ordained among them to keep their Sabbath on the first day of the week: therefore to keep the Sabbath on that day is the very commandment of the Lord; and how can he be either a true prophet, or have any grace of God's Spirit in his heart, who, seeing so clearly the Lord's day to have been instituted and ordained by the apostles, will not acknowledge the keeping holy of the Lord's day to be a commandment of the Lord?
The Jews confess this change of the Sabbath to have been made by the apostles (Peter Alphon in Dialog, contra Judæos, tit.12); they are therefore more blind and sottish than the Jews, who profanely deny it.
At Troas likewise St. Paul, together with seven of the chief evangelists of the church, Sopater, Aristarchus, Secundus, Gaius, Timotheus, Tychicus, and Trophimus (Acts xx.4, 5, &c.), and all the Christians that were there, kept the holy Sabbath on the first day of the week, in praying, preaching, and receiving the Lord's Supper.
And it is a thing to be noted, that Luke saith not, that the disciples were sent to hear Paul preach, but the disciples being come together to break bread upon the first day of the week, that is, to be partakers of the holy communion, at what time the Lord's death was by the preaching of the word shewed (1 Cor. xviii.26), Paul preached unto them, &c. And that none kept those meetings but Christians, who are called disciples (Acts xi.26.) But at Philippi, where as yet there was no disciples, Paul is said to go on their Sabbath day to the place where the Jews and their proselytes were wont to pray, and there preached unto them (Acts xvi.12, 13.) So that it is as clear as the sun, that it was the Christians' usual manner to pass over the Jewish seventh day (Acts xxi.4, &c.), and to keep the Sabbath and their holy meetings on the first day of the week. And why does St. John call this the Lord's day, but because it was a day known to be generally kept holy throughout all the churches which the apostles planted to the honour of the Lord Jesus, who rose from death to life upon that day; which St. John called the Lord's day (Rev. i.10), the rather to stir up Christians to a thankful remembrance of their redemption by Christ's resurrection from the dead. And with the day, the blessing of the Sabbath is likewise translated to the Lord's day; because all the sanctification belonging to this new world is in Christ (Heb. ii.5), and from him conveyed to Christians: and because there cannot come a greater authority than that of Christ and his apostles, nor the like cause as the new creation of the world; therefore the Sabbath can never be altered from this day to any other whilst this world lasts. Add to this, how the Scripture notes, that in the first planting and settling of the church, nothing was done but by the special order and direction of the apostles (1 Cor. xi.34; xiv.36, 37; Tit. i.5; Acts xv.6, 24), and the apostles did nothing but what they had warrant for from Christ (1 Cor. xi.23.)
To sanctify, then, the Sabbath on the seventh day is not a ceremonial law abrogated, but the moral and perpetual law of God perfected; so that the same perpetual commandment which bound the Jews to keep the Sabbath on that seventh day, to celebrate the world's creation, binds Christians to solemnize the Sabbath on this seventh day, in memorial of the world's redemption; for the fourth commandment being a moral law, requires a seventh day to be kept holy for ever. And the morality of this, as of the rest of the commandments, is more religiously to be kept of us under the gospel, than of the Jews under the law; by how much we, in baptism, have made a more special covenant with God to keep his commandments; and God has covenanted with us to free us from the curse, and to assist us with his Spirit to keep his laws. And that this commandment of the Sabbath, as well as the other nine, is moral and perpetual, may plainly appear by these reasons: --