CCC 2174 - Jesus rose from the dead "on the first day of the week." Because it is the "first day," the day of Christ's Resurrection recalls the first creation. Because it is the "eighth day" following the sabbath, it symbolizes the new creation ushered in by Christ's Resurrection. For Christians it has become the first of all days, the first of all feasts, the Lord's Day (he kuriake hemera, dies dominica)—Sunday:
Sunday—fulfillment of the sabbath
CCC 2175 - Sunday is expressly distinguished from the sabbath which it follows chronologically every week; for Christians its ceremonial observance replaces that of the sabbath. In Christ's Passover, Sunday fulfills the spiritual truth of the Jewish sabbath and announces man's eternal rest in God. For worship under the Law prepared for the mystery of Christ, and what was done there prefigured some aspects of Christ:
Those who lived according to the old order of things have come to a new hope, no longer keeping the sabbath, but the Lord's Day, in which our life is blessed by him and by his death.
CCC 2176 - The celebration of Sunday observes the moral commandment inscribed by nature in the human heart to render to God an outward, visible, public, and regular worship "as a sign of his universal beneficence to all." Sunday worship fulfills the moral command of the Old Covenant, taking up its rhythm and spirit in the weekly celebration of the Creator and Redeemer of his people
Didache, between 80 and 140 AD
"But every Lord’s Day, gather yourselves together, and break bread, and give thanksgiving after having confessed your transgressions, so that you sacrifice may be pure.”
Ignatius, about 107 AD
“If, therefore, those who were brought up in the ancient order of things have come to the possession of a new hope, no longer observing the Sabbath, but living in the observance of the Lord's Day, on which also our life has sprung up again in Him...Let us therefore no longer keep the Sabbath after the Jewish manner, and rejoice in the days of idleness; for "he that does not work, let him not eat." ...let every friend of Christ keep the Lord's day as a festival, the resurrection day, the queen and chief of all the days [of the week]”
Aristides, about 125 AD
“However, [the Jews,] too have erred from true knowledge. In their imagination, they think that it is God whom they serve. Actually, by their type of worship, they render their service to the angels and not to God. For example, they do this when they celebrate Sabbaths.”
Justin Martyr, about 160 AD
“Is there any other matter my [Jewish] friends, in which we Christians are blamed, than this: that we do not live after the Law ... and do not observe Sabbaths, as you do?”
“There was no need of circumcision before Abraham. Nor was there need of the observance of Sabbaths, or of feasts and sacrifices, before Moses. Accordingly, there is no more need of them now.”
“And on the day called Sunday, all who live in cities or in the country gather together to one place. And the memoirs of the apostles or the writings of the prophets are read, as long as time permits. Then, when the reader has ceased, the president verbally instructs us and exhorts us to imitate these good things. Then we all rise together and pray. And, as we said before, when our prayer is ended, bread and wine and water are brought. Then, the president in like manner offers prayers and thanksgivings, according to his ability. And the people assent, saying “Amen.” Then, [the Lord’s Supper] is distributed to everyone, and everyone participates in [the bread and wine], over which thanks has been given. And a portion of it is sent by the deacons to those who are absent. ... But Sunday is the day on which we all hold our common assembly, because it is the first day on which God ... made the world. And Jesus Christ our Savior rose from the dead on that same day.”
Eusebius, citing Irenaeus, about 180 AD
"This custom of not bending the knee on Sunday is a symbol of the resurrection, through which we have been set free by the grace of Christ.”
Tertullian, about 197 AD
“We devote Sunday to rejoicing for a far different reason than sun worship. ... Others ... suppose that the sun is the god of the Christians ... because we make Sunday a day of festivity.”
“Just as the abolition of fleshly circumcision and of the old Law is demonstrated as having been consummated at its specific times, so also the observance of the Sabbath is demonstrated to have been temporary.”
Anatolius, about 270 AD
“It should not be lawful to celebrate the Lord’s mystery of Easter [i.e. the Lord’s Supper] at any other time but on the Lord’s Day, the day on which the Lord’s resurrection from death took place.”
Victorinus, about 280 AD
“And let this become a rigorous fast, lest we should appear to observe any Sabbath with the Jews. For concerning [their Sabbath], Christ himself, the Lord of the Sabbath, says by His prophets that “His soul hates.” In His body, He abolished this Sabbath.”
The Council of Elvira (300) decreed: "If anyone in the city neglects to come to church for three Sundays, let him be excommunicated for a short time so that he may be corrected"
Epistle of Barnabas (xv) we read: "Wherefore, also, we keep the eight day (i.e. the first of he week) with joyfulness, the day also on which Jesus rose again from the dead".
The evidence is historically solid that early Christians, long before Constantine, worshiped on the first day of the week and not on the seventh day. Constantine could not have instituted a new practice since the practice predates him.
However, I was curious what Constantine decreed that cause people like yourself to think that he changed the day of worship. What I found was that on March 7, 321 AD Constantine issued this decree: “On the venerable Day of the Sun let the magistrates and people residing in cities rest, and let all workshops
be closed. In the country, however, persons engaged in agriculture may freely and lawfully continue their pursuits.” The decree doesn't mention anything in regards to activities of the church. Essentially it is the first "blue Sunday" law or "Sunday closure" law. The only reason a Sabbatarian would think that this dealt with worship is because the Jewish Sabbath day was celebrated by resting. Christians do not worship by resting because our rest comes later. "There remains therefore a rest for the people of God. For he who has entered His rest has himself also ceased from his works as God did from His. Let us therefore be diligent to enter that rest, lest anyone fall according to the same example of disobedience" (Hebrews 4:9-11). In addition, Constantine's decree only applied to city dwellers; farmers were specifically exempted from the law.
s Sabbath keepers falsely teach. Christians never kept the Sabbath from the apostolic age (33 AD) through the time of Constantine (325AD). Constantine merely made the first "Sunday closure law", since it had already been the day Christians worship for 300 years!
The practice of meeting together on the first day of the week for the celebration of the Eucharistic Sacrifice is indicated in Acts, xx 7; I Cor., xvi, 2; in Apoc., i, 10, it is called the Lord's day. In the Didache (xiv) the injunction is given: "On the Lord's Day come together and break bread. And give thanks (offer the Eucharist),
St. Ignatius (Ep. ad Magnes. ix) speaks of Christians as "no longer observing the Sabbath, but living in the observance of the Lord's Day, on which also Our Life rose again". In the tEpistle of Barnabas (xv) we read: "Wherefore, also, we keep the eight day (i.e. the first of he week) with joyfulness, the day also on which Jesus rose again from the dead".
St. Justin is the first Christian writer to call the day Sunday (I Apol., lxvii) in the celebrated passage in which he describes the worship offered by the early Christians on that day to God. The fact that they ment together and offered public worship on Sunday necessitated a certain rest from work on that day. However, Tertullian (202) is the first writer who expressly mentions the Sunday rest: "We, however (just as tradition has taught us), on the day of the Lord's Resurrection ought to guard not only against kneeling, but every posture and office of solicitude, deferring even our businesses lest we give any place to the devil" ("De orat.", xxiii; cf. "Ad nation.", I, xiii; "Apolog.", xvi).
The Council of Elvira (300) decreed: "If anyone in the city neglects to come to church for three Sundays, let him be excommunicated for a short time so that he may be corrected"
St. Cæsarius of Arles in the sixth century teaching that the holy Doctors of the Church had decreed that the whole glory of the Jewish Sabbath had been transferred to the Sunday, and that Christians must keep the Sunday holy in the same way as the Jews had been commanded to keep holy the Sabbath Day