The following is a brief presentation on the Lord’s Day written by Dave Melton.  With his permission I am publishing it to Armchair Theologian.  I have formatted the text to make it easy on the eyes but all commentary is his.  I’ll offer my own thoughts at the end separately.

“I was in the Spirit on the Lord’s day, and I heard behind me a loud voice like the sound of a trumpet,” Revelation 1:10 NASB

The problem in answering this question is that the expression “the Lord’s day” (“τῇ κυριακῇ ἡμέρᾳ”) is a hapax logomenon (a word that occurs only once in an author’s writings or a text). When a word is only used once it is difficult, if not impossible, to infer the writer’s meaning, since there are no other examples of word usage to compare. Doctrine should not be built on a hapax legomenon .
As such, you cannot use Scripture to interpret Scripture when this occurs. It is one of the rare instances where you must look outside of Scriptures to find the answer. The short answer is that “the Lord’s day” is the first day of the week when early Christians celebrated Jesus’ resurrection. This is attested to by Ignatius of Antioch and Justin Marytr.  Ignatius of Antioch also known as Ignatius Theophorus was a student of the Apostle John.

Epistle to the Magnesians, Chapter 9:

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.
If, then, those who were conversant with the ancient Scriptures came to newness of hope, expecting the coming of Christ, as the Lord teaches us when He says, “If ye had believed Moses, ye would have believed Me, for he wrote of Me;” and again, “Your father Abraham rejoiced to see My day, and he saw it, and was glad; for before Abraham was, I am;” how shall we be able to live without Him? The prophets were His servants, and foresaw Him by the Spirit, and waited for Him as their Teacher, and expected Him as their Lord and Saviour, saying, “He will come and save us.” Let us therefore no longer keep the Sabbath after the Jewish manner, and rejoice in days of idleness; for “he that does not work, let him not eat.” For say the [holy] oracles, “In the sweat of thy face shalt thou eat thy bread.” But let every one of you keep the Sabbath after a spiritual manner, rejoicing in meditation on the law, not in relaxation of the body, admiring the workmanship of God, and not eating things prepared the day before, nor using lukewarm drinks, and walking within a prescribed space, nor finding delight in dancing and plaudits which have no sense in them. And after the observance of the Sabbath, 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]. Looking forward to this, the prophet declared, “To the end, for the eighth day,” on which our life both sprang up again, and the victory over death was obtained in Christ, whom the children of perdition, the enemies of the Saviour, deny, “whose god is their belly, who mind earthly things,” who are “lovers of pleasure, and not lovers of God, having a form of godliness, but denying the power thereof.” These make merchandise of Christ, corrupting His word, and giving up Jesus to sale: they are corrupters of women, and covetous of other men’s possessions, swallowing up wealth insatiably; from whom may ye be delivered by the mercy of God through our Lord Jesus Christ!

Ignatius also had a student, Justin Martyr. Justin Martyr wrote in his First Apology,

Weekly worship of the Christians, Chapter 67:
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, 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.

The idea is that we don’t know from scripture what the Lord’s Day is, it’s not defined by scripture.  We can see though that the early church believed the Lord’s Day was Sunday.  It was a matter of tradition.  It would be reasonable to conclude that John likely did mean Sunday when he referenced the Lord’s Day in revelation.  He would have been aware of the teachings above and if there had been any controversy it’s logical to assume he would have clarified.  But there wasn’t and he didn’t.

Does this mean Sunday is the new Sabbath?  No not really, but it is fair to assert based on history that the Lord’s Day is Sunday.  This is an ancient Christian tradition, and the day itself is certainly made holy by the preaching of the Word on that day, as it would on any other.