"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 by Him and by His death -- whom some deny, by which mystery we have obtained faith, and therefore endure, that we may be found the disciples of Jesus Christ, our only Master -- how shall we be able to live apart from Him, whose disciples the prophets themselves in the Spirit did wait for Him as their Teacher? And therefore He whom they rightly waited for, being come, raised them from the dead." ... Be ye salted in Him, lest any one among you should be corrupted, since by your savour ye shall be convicted. It is absurd to profess Christ Jesus, and to Judaize. For Christianity did not embrace Judaism, but Judaism Christianity, that so every tongue which believeth might be gathered together to God."

From the first century Didache, Chapter 14, "Christian Assembly on the Lord's Day":

"But every Lord's day gather yourselves together, and break bread, and give thanksgiving after having confessed your transgressions, that your sacrifice may be pure. But let no one who is at odds with his fellow come together with you, until they be reconciled, that your sacrifice may not be profaned. For this is that which was spoken by the Lord: "In every place and time offer to me a pure sacrifice; for I am a great King, says the Lord, and my name is wonderful among the nations."

Letter of Barnabas 15:6-8, written in A.D. 74, at the end of a few paragraphs on the old Jewish Shabat vs. the Christian Lord's Day, wrote:

"We keep the eighth day [Sunday] with joyfulness, the day also on which Jesus rose again from the dead."

And what about Justin Martyr, a Palestinian (born around the year A.D. 100) who converted to Christianity in about A.D.130 and was martyred in A.D. 165 .? He wrote: "On the day called the Feast of the Sun, all who live in towns or in the country assemble in one place, and the memoirs of the Apostles or the writings of the Prophets are read as time permits. Then, when the reader has ended, the President instructs and encourages the people to practice the truths contained in the Scripture lections. Thereafter, we all stand up and offer prayers together ... Our prayers being ended, we salute one another with a kiss. Then bread, and a cup of wine mixed with water, are brought to him who presides over the brethren. He, taking them, offers praise and glory to the Father of all through the Name of the Son and the Holy Spirit, and giving thanks at great length for that we have been counted worthy to receive these gifts from God; and when he finishes the prayers and thanksgivings all the people present cry aloud, Amen ... After the President [the presiding priest] has given thanks and all the people have said Amen, those among us who are called deacons give to all present ... the bread and wine mixed with water over which thanks has been given, and carry it also to those who are absent. And this food is called Eucharist by us, of which it is not right for any one to partake save only he who believes that the things taught by us are true, and is washed with the washing that is for the forgiveness of sins and regeneration, and so lives as Christ commanded us."

The Scripture below also supports these early Catholics. Your options are to believe that either the New Testament and the early Church are wrong or that the Catholic Church is still right about Sunday worship. Besides, "Sabbath" means "Rest" (from the word for "cease"), not "Saturday" or "seven" or "week," so even if the Mosaic Laws were binding on Christians (which they are not, aside from those aspects rooted in Natural Law and God's Eternal Law, which are always binding), there is no need for the "Sabbath" (day of rest) to be kept on Saturdays instead of Sundays. Christians simply keep Sunday as a day of rest, and we keep it holy by attending Mass and refraining from unecessary servile work. In this way, we, in a sense, "keep Sabbath" as in a day of rest -- but not the Mosaic or modern Jewish Sabbath because we are freed from the Law.

2 Corinthians 3:7-11:
But if the ministration of death, written and engraven in stones, was glorious, so that the children of Israel could not stedfastly behold the face of Moses for the glory of his countenance; which glory was to be done away: How shall not the ministration of the spirit be rather glorious? For if the ministration of condemnation be glory, much more doth the ministration of righteousness exceed in glory. For even that which was made glorious had no glory in this respect, by reason of the glory that excelleth. For if that which is done away was glorious, much more that which remaineth is glorious.

We celebrate the Lord's Day, the day when Jesus Christ walked out of His tomb and the fruits of fulfillment of the Old Testament prophecies were made clear.

We are no longer under the Old Covenant Law, but if you still want to stick with Shabat, why not go all the way and convert to Judaism? Read Leviticus and follow all the Old Covenant laws. If you're a woman, be sure not to spoil the family's furniture by sitting on it while menstruating (Leviticus 15:20) --- and make sure your husband ritually cleans himself if he forgets and touches your bed at this time! If you're a man and sleep with your bondmaid, please remember to bring a ram to the door of the tabernacle (Leviticus 19:20-21).

 

Footnote:
Even the Jews' Talmud acknowledges that Christians worship on Sunday. From Abhodah Zarah, 6a, we read: "He is called a Christian who follows the false teachings of that man [Jesus Christ], who taught them to celebrate the feast on the first day of the Sabbath, that is, to worship on the first day after the Sabbath." So any Messianics and Judaized Christians out there who appeal to the Gospel on one hand, and to rabbinical "authority" on the other, take heed.

Relevant Scripture

Revelation 1:10
I was in the Spirit on the Lord's day, and heard behind me a great voice, as of a trumpet

Further Reading

Sunday, the Sabbath, and the Mark of the Beast