Why Was the Jewish Sabbath Changed to Sunday?
The Lord’s Day
Unlike the Jews, Catholics have always worshiped God on Sunday which is “the first day of the week” called “the Lord’s Day”.
Said another way, Catholics have never worshiped God on the Jewish Sabbath, which is Saturday and the last day of the week.
Sunday is never called “the Sabbath” by the ancient Fathers and historians of the Catholic Church, but “the Lord’s day” (kuriake).
Proof from Scripture
The practice of meeting together on the first day of the week for attending the Holy Sacrifice of the Mass is indicated in the following Scripture sources.
“And on the first day of the week [Sunday], when we were assembled to break bread
[an euphemism to mean “the Mass” due to the “Discipline of the Secret” as explained below *],Paul discoursed with them, being to depart on the morrow: and he continued his speech until midnight.” (Acts of the Apostles 20:7.)
“On the first day of the week [Sunday] let every one of you put apart with himself, laying up what it shall well please him; that when I come, the collections be not then to be made.” (1 Corinthians 16:2.)
The Lord’s Day
Sunday is also called “The Lord’s day”
“I was in the spirit on the Lord’s day.”
Proof from Other Sources
From the beginning, Catholics have observed the Lord’s Day on Sunday, not on the Jewish Saturday Sabbath as proven in a First Century A.D. source.
Patriarch Saint Ignatius of Antioch
“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 rose again.” (Patriarch Saint Ignatius of Antioch, a.k.a. Theophorus
[ho Theophoros], [b. in Syria c. 50 A.D. - d. Martyred in the Flavian amphitheater
at Rome, Italy on Tuesday, February 1, 107 A.D.], Patriarch of Antioch,
Martyr, an Apostolic Father of the Catholic Church, appointed the Bishop
of Antioch by Saint Peter the Apostle and Consecrated a Bishop by an Apostle,
most probably either by Saint Peter or by Saint John the Apostle of whom
he had been a Disciple, “Epistle to the Magnesians”, Chapter IX , Let
Us Live with Christ.)
Jesus Christ taught His Apostles and Disciples to use Sunday for the Lord’s Day.
“On the day called Sunday, all who live in cities or in the country gather together to one place... Sunday is the day on which we all hold our common assembly [the Mass], because it is the first day on which God, having wrought a change in the darkness and matter, made the world; and Jesus Christ our Saviour on the same day rose from the dead. For He was crucified on the day before [Friday] that of Saturn (Saturday); and on the day after [Sunday] that of Saturn, which is the day of the Sun [Sunday], having appeared to His Apostles and Disciples, He [Jesus Christ] taught them [His Apostles and Disciples] these things, which We have submitted to you also for your consideration.” (Saint Justin Martyr [b. Sichem, a.k.a. Neapolis, or Flavia Neapolis, modern day Nablus, Palestine c. 100 A.D. - d. Rome, Italy c. 165 A.D.], “The First Apology of Justin”, Chapter LXVII , Weekly Worship of the Christians; emphasis added.)
“Others... suppose that the sun is the god of the Christians, because it is a well known fact that we pray towards the East, or because we make Sunday a day of festivity.” (Tertullian, Quintus Septimius Florens Tertullianus [b. Carthage, 155 A.D. - d. 225 A.D.], Part I. “Apologetics”, Section VI , “To The Nations”, Chapter XIII , The Charge of Worshiping the Sun Met by a Retort; emphasis added.)
Bishop Saint Augustine
“What you call Sunday, we call ‘the Lord’s day’, and on it we do not worship the sun, but the Lord’s resurrection.” (Bishop Saint Augustine, a.k.a. Aurelius Augustinus [b. Tagaste, Africa, Saturday, November 13, 354 A.D. - d. Hippo Regia, Africa, Wednesday, August 28, 430 A.D.], Bishop of Hippo Regia, Father and Doctor of the Catholic Church, “Concerning Two Souls, Against the Manichaeans”, [De Duabus Animabus Contra Manichaeos.] 391 A.D., Book XVIII , The Relation of Christ to Prophecy, Continued, ¶ 5; emphasis added.)
A short treatise, which was considered by some of the Fathers of the Catholic Church as next to the Holy Scriptures in importance, gives the following injunction, which also explains the need to confess one’s sins.
“On the Lord’s Day come together and break bread [i.e. attend the Mass]. And give thanks after confessing your sins that your Sacrifice [of the Mass] may be pure.” (DIDACHE [“Doctrine of the Twelve Apostles”], written circa 80 A.D. - 100 A.D., Chapter XIV  Christian Assembly on the Lord’s Day; emphasis added.)
Patriarch Saint Athanasius
“There are other books besides these not indeed included in the Canon [of Sacred Scripture], but appointed by the Fathers [of the Catholic Church] to be read by those who have lately approached the Church and are eager to be instructed and to learn pious doctrine... [including] that which is called the ‘Teaching of the Apostles’.” (Patriarch Saint Athanasius [b. Alexandria, Egypt 296 A.D. - d. Alexandria, Egypt on Wednesday, May 2, 373A.D.], Patriarch of Alexandria, Egypt, Father of Orthodoxy in the Catholic Church, “Epistle 39", ¶ 7.)
“Wherefore, also, we keep the eighth day [i.e. Sunday, the first day of the week] with joyfulness, the day also on which Jesus rose again from the dead.” (“Epistle of Barnabas”, Chapter XV , The False and the True Sabbath, written circa 96 A.D. or 98 A.D. The “Epistle of Barnabas” is found after the books of the New Testament in the famous “Codex Sinaiticus” [c. 340 A.D.] which is the second oldest known Catholic Bible in existence.)
John Ambrose McHugh, O.P., S.T.M., Litt.D.
Charles Jerome Callan, O.P., S.T.M., Litt.D.
Imprimatur: + Patrick Cardinal Joseph Hayes, D.D.
Imprimatur Date: Wednesday, January 3, 1923
“This Commandment about the observance of the [Jewish] Sabbath, on the other hand, considered as to the time appointed for its fulfilment, is not fixed and unalterable, but susceptible of change, and belongs not to the moral, but the ceremonial law. Neither is it a principle of the natural law; we are not instructed by nature to give external worship to God on that day, rather than on any other. And in fact the [Jewish] Sabbath was kept holy only from the time of the liberation of the people of Israel from the bondage of Pharaoh. The observance of the [Jewish] Sabbath was to be abrogated at the same time as the other Hebrew rites and ceremonies, that is, at the death of Christ. Having been, as it were, images which foreshadowed the light and the truth, these ceremonies were to disappear at the coming of that light and truth, which is Jesus Christ. Hence St. Paul, in his Epistle to the Galatians, when reproving the observers of the Mosaic rites, says: ‘You observe days, and months, and times, and years. I am afraid of you, lest perhaps I have laboured in vain amongst you.’ [Galatians 4:10-11]. And he writes to the same effect to the Colossians.” (“The Catechism of the Council of Trent for Parish Priests”, a.k.a. “The Roman Catechism”, Issued by Order of Pope Saint Pius V, Antonio-Michele Ghislieri [Friday, January 7, 1566 - Monday, May 1, 1572]. Translated by John Ambrose McHugh, O.P., S.T.M., Litt.D., and Charles Jerome Callan, O.P., S.T.M., Litt.D. Imprimatur: + Patrick Cardinal Joseph Hayes, D.D., [b. at New York, New York on Wednesday, November 20, 1867 - d. at New York, New York on Sunday, September 4, 1938;, Archbishop of New York, Monday, March 10, 1919 - Sunday, September 4, 1938]. “Part III : The Decalogue, The Promulgation of the Law, Third Commandment, How The Third Differs From The Other Commandments”, pp. 397-398; emphasis added.)
Based on the above data, Sunday is called “the first day of the week” and “the Lord’s Day” in the New Testament. Christ Himself taught His Apostles and Disciples to use Sunday for “the Lord’s Day” of the New and Eternal Covenant. It was on a Sunday that Jesus Christ rose from the dead, and on Pentecost Sunday that Jesus Christ sent down God the Holy Ghost. That first Pentecost Sunday was the birthday of the Catholic Church. The observance of the Saturday Jewish Sabbath was abolished along with the other Jewish rites and ceremonies at the death of Jesus Christ because the Old Covenant Law had only the “shadow of the good things to come, not the very image of the things.” (Hebrews 10:1.)
History of the Church
“It was not lawful for those initiated in the doctrine of Christianity to speak of, or publicly represent certain of its tenets and usages, for example ... the Holy Eucharist and the Sacrifice of the Mass... Hence also the exclusion of even the Catechumens from the ‘Mass of the Faithful’. This caution the early Christians were compelled to observe to guard themselves against intruders among the Catechumens and their Holy Faith against base misrepresentations. The ‘Discipline of the Secret’, which lasted during the first five centuries [A.D.], accounts for the guarded language of the early writers... and explains why the Fathers of [the Catholic Church during] this period write so cautiously concerning the Real Presence of our Lord in the [Sacrament of the Most Holy] Eucharist.” (Rev. J. A Birkhæuser, “History of the Church”, Fr. Pustet & Co., 1893, page 118, ¶ 309; emphasis added).