Recently I was discussing this topic with several friends and family. If the Jews celebrated the Sabbath on Saturday, why do we Christians celebrate it on Sunday? I have copied a post straight from New Advent online Dictionary.
Sunday (Day of the Sun), as the name of the first day of the week, is derived from Egyptian astrology. The seven planets, known to us as Saturn, Jupiter, Mars, the Sun, Venus, Mercury, and the Moon, each had an hour of the day assigned to them, and the planet which was regent during the first hour of any day of the week gave its name to that day (see CALENDAR). During the first and second century the week of seven days was introduced into Rome from Egypt, and the Roman names of the planets were given to each successive day. The Teutonic nations seem to have adopted the week as a division of time from the Romans, but they changed the Roman names into those of corresponding Teutonic deities. Hence the dies Solis became Sunday (German, Sonntag). Sunday was the first day of the week according to the Jewish method of reckoning, but for Christians it began to take the place of the Jewish Sabbath in Apostolic times as the day set apart for the public and solemn worship of God. 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), after confessing your sins that your sacrifice may be pure". 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 Epistle of Barnabas (xv) we read: "Wherefore, also, we keep the eight day (i. e. the first of the 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).
These and similar indications show that during the first three centuries practice and tradition had consecrated the Sunday to the public worship of God by the hearing of the Mass and the resting from work. With the opening of the fourth century positive legislation, both ecclesiastical and civil, began to make these duties more definite. 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" (xxi). In the Apostolic Constitutions, which belong to the end of the fourth century, both the hearing of the Mass and the rest from work are prescribed, and the precept is attributed to the Apostles. The express teaching of Christ and St. Paul prevented the early Christians from falling into the excesses of Jewish Sabbatarianism in the observance of the Sunday, and yet we find 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. He especially insisted on the people hearing the whole of the Mass and not leaving the church after the Epistle and the Gospel had been read. He taught them that they should come to Vespers and spend the rest of the day in pious reading and prayer. As with the Jewish Sabbath, the observance of the Christian Sunday began with sundown on Saturday and lasted till the same time on Sunday. Until quite recent times some theologians taught that there was an obligation under pain of venial sin of assisting at vespers as well as of hearing Mass, but the opinion rests on no certain foundation and is now commonly abandoned. The common opinion maintains that, while it is highly becoming to be present at Vespers on Sunday, there is no strict obligation to be present. The method of reckoning the Sunday from sunset to sunset continued in some places down to the seventeenth century, but in general since the Middle Ages the reckoning from midnight to midnight has been followed. When the parochial system was introduced, the laity were taught that they must hear Mass and the preaching of the Word of God on Sundays in their parish church. However, toward the end of the thirteenth century, the friars began to teach that the precept of hearing Mass might be fulfilled by hearing it in their churches, and after long and severe struggles this was expressly allowed by the Holy See. Nowadays, the precept may be fulfilled by hearing Mass in any place except a strictly private oratory, and provided Mass is not celebrated on a portable altar by a privilege which is merely personal.
The obligation of rest from work on Sunday remained somewhat indefinite for several centuries. A Council of Laodicea, held toward the end of the fourth century, was content to prescribe that on the Lord's Day the faithful were to abstain from work as far as possible. At the beginning of the sixth century St. Caesarius, as we have seen, and others showed an inclination to apply the law of the Jewish Sabbath to the observance of the Christian Sunday. The Council held at Orleans in 538 reprobated this tendency as Jewish and non-Christian. From the eight century the law began to be formulated as it exists at eh present day, and the local councils forbade servile work, public buying and selling, pleading in the law courts, and the public and solemn taking of oaths. There is a large body of civil legislation on the Sunday rest side by side with the ecclesiastical. It begins with an Edict of Constantine, the first Christian emperor, who forbade judges to sit and townspeople to work on Sunday. He made an exception in favour of agriculture. The breaking of the law of Sunday rest was punished by the Anglo-Saxon legislation in England like other crimes and misdemeanours. After the Reformation, under Puritan influence, many laws were passed in England whose effect is still visible in the stringency of the English Sabbath. Still more is this the case in Scotland. There is no federal legislation in the United States on the observance of the Sunday, but nearly all the states of the Union have statues tending to repress unnecessary labour and to restrain the liquor traffic. In other respects the legislation of the different states on this matter exhibits considerable variety. On the continent of Europe in recent years there have been several laws passed in direction of enforcing the observance of Sunday rest for the benefit of workmen.