14. And on the Lord’s own day gather yourselves together and break bread and give thanks, first confessing your transgressions, that your sacrifice may be pure. 2And let no man, having his dispute with his fellow, join your assembly until they have been reconciled, that your sacrifice may not be defiled; 3for this sacrifice it is that was spoken of by the Lord; In every place and at every time offer Me a pure sacrifice; for I am a great king, saith the Lord, and My name is wonderful among the nations.

First, Ignatius in his Epistle to the Magnesians (about A.D. 110–117) stressed the importance of Lord’s Day by contrasting the worship done on that day with that formerly observed on the Sabbath

And we afterwards continually remind each other of these things. And the wealthy among us help the needy; and we always keep together; and for all things wherewith we are supplied, we bless the Maker of all through His Son Jesus Christ, and through the Holy Ghost. 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, and exhorts to the imitation of these good things. Then we all rise together and pray, and, as we before said, when our prayer is ended, bread and wine and water are brought, and the president in like manner offers prayers and thanksgivings, according to his ability,2 and the people assent, saying Amen; and there is a distribution to each, and a participation of that over which thanks have been given, and to those who are absent a portion is sent by the deacons. And they who are well to do, and willing, give what each thinks fit; and what is collected is deposited with the president, who succours the orphans and widows, and those who, through sickness or any other cause, are in want, and those who are in bonds, and the strangers sojourning among us, and in a word takes care of all who are in need. But Sunday is the day on which we all hold our common assembly, 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 that of Saturn (Saturday); and on the day after that of Saturn, which is the day of the Sun, having appeared to His apostles and disciples, He taught them these things, which we have submitted to you also for your consideration.