This is the result of the research I did to answer that friend. Below are all of the relevant verses and references from the early church (3rd century or earlier) that I found. There could be some significant verses out there that I didn't find.
Here are my conclusions:
1. The phrase "day of the Lord" refers to the future day of the Lord's judgment.
2. The phrase "Lord's Day" refers to Sunday or the eighth day of the week.
3. The early church felt it was extremely important to meet on the first day of the week (the Lord's Day) to celebrate the resurrection of Jesus.
4. It appears that calling the first day of the week the Lord's Day was a later development that occurred prior to John's writing of Revelation but after the rest of the writing of the rest of the New Testament.
5. Referring to the Lord's day as the eighth day of the week like many of the church fathers did hearkens back to the creation account. On the seventh day God rested. On the eighth he brought about his work of redemption.
One stream of thought ran through many of the church fathers was that we should not fast or kneel on the Lord's Day. I found this extremely interesting. This stream of thought seems to stem from Matthew 9:14-17
Matthew 9:14 Then the disciples of John came to him, saying, "Why do we and the Pharisees fast often, but your disciples do not fast?" 15 And Jesus said to them, "The wedding guests cannot mourn as long as the bridegroom is with them, can they? The days will come when the bridegroom is taken away from them, and then they will fast. 16 No one sews a piece of unshrunk cloth on an old cloak, for the patch pulls away from the cloak, and a worse tear is made. 17 Neither is new wine put into old wineskins; otherwise, the skins burst, and the wine is spilled, and the skins are destroyed; but new wine is put into fresh wineskins, and so both are preserved."
The early church believed that Jesus was present during the Lord's Supper, which was the centerpiece of the gathering on the Lord's Day. With Jesus being present there was no reason to fast or kneel. The church was to celebrate the resurrection of our king.
"day of the Lord" references
Isaiah 58:13
If you keep your feet from breaking the Sabbath and from doing as you please on my holy day, if you call the Sabbath a delight and the Lord's holy day honorable, and if you honor it by not going your own way and not doing as you please or speaking idle words.
Isaiah 61:2
to proclaim the year of the Lord's favor and the day of vengeance of our God, to comfort all who mourn.
Lamentations 2:22
"As you summon to a feast day, so you summoned against me terrors on every side. In the day of the Lord's anger no one escaped or survived; those I cared for and reared, my enemy has destroyed."
Ezekiel 7:19
They will throw their silver into the streets, and their gold will be an unclean thing. Their silver and gold will not be able to save them in the day of the Lord's wrath. They will not satisfy their hunger or fill their stomachs with it, for it has made them stumble into sin.
Zep 1:8
On the day of the Lord's sacrifice I will punish the princes and the king's sons and all those clad in foreign clothes.
Zep 1:18
Neither their silver nor their gold will be able to save them on the day of the Lord's wrath. In the fire of his jealousy the whole world will be consumed, for he will make a sudden end of all who live in the earth."
Zep 2:2
before the appointed time arrives and that day sweeps on like chaff, before the fierce anger of the Lord comes upon you, before the day of the Lord's wrath comes upon you.
Zep 2:3
Seek the Lord, all you humble of the land, you who do what he commands. Seek righteousness, seek humility; perhaps you will be sheltered on the day of the Lord's anger.
Ac 2:20
The sun shall be turned into darkness, And the moon into blood, Before the coming of the great and awesome day of the Lord.
1Co 1:8
who will also confirm you to the end, that you may be blameless in the day of our Lord Jesus Christ.
1Co 5:5
deliver such a one to Satan for the destruction of the flesh, that his spirit may be saved in the day of the Lord Jesus.
2Co 1:13-15
For we are not writing any other things to you than what you read or understand. Now I trust you will understand, even to the end (as also you have understood us in part), that we are your boast as you also are ours, in the day of the Lord Jesus.
2Pe 3:10
But the day of the Lord will come as a thief in the night, in which the heavens will pass away with a great noise, and the elements will melt with fervent heat; both the earth and the works that are in it will be burned up.
1Th 5:2
For you yourselves know perfectly that the day of the Lord so comes as a thief in the night.
the only "Lord's day" references
Re 1:10
On the Lord's Day I was in the Spirit, and I heard behind me a loud voice like a trumpet.
"first day of the week" references
Mt 28:1
After the Sabbath, at dawn on the first day of the week, Mary Magdalene and the other Mary went to look at the tomb.
Mr 16:2
Very early on the first day of the week, just after sunrise, they were on their way to the tomb.
Mr 16:9
When Jesus rose early on the first day of the week, he appeared first to Mary Magdalene, out of whom he had driven seven demons.
Lu 24:1
On the first day of the week, very early in the morning, the women took the spices they had prepared and went to the tomb.
Joh 20:1
Early on the first day of the week, while it was still dark, Mary Magdalene went to the tomb and saw that the stone had been removed from the entrance.
Joh 20:19
On the evening of that first day of the week, when the disciples were together, with the doors locked for fear of the Jews, Jesus came and stood among them and said, Peace be with you!"
Ac 20:7
On the first day of the week we came together to break bread. Paul spoke to the people and, because he intended to leave the next day, kept on talking until midnight.
1Co 16:2
On the first day of every week, each one of you should set aside a sum of money in keeping with his income, saving it up, so that when I come no collections will have to be made. And here are the early church references to the phrase "Lord's Day"
writings from the early church
From the epistle of Ignatius to the Magnesians:
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 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 Savior, 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.”
From the epistle of Ignatius to the Trallians:
At the dawning of the Lord’s day He arose from the dead, according to what was spoken by Himself, “As Jonah was three days and three nights in the whale’s belly, so shall the Son of man also be three days and three nights in the heart of the earth.” The day of the preparation, then, comprises the passion; the Sabbath embraces the burial; the Lord’s Day contains the resurrection.
From the epistle of Ignatius to the Phillipians:
If any one fasts on the Lord’s Day or on the Sabbath, except on the paschal Sabbath only, he is a murderer of Christ.
from the fragments of the lost writings of Iraneus:
This [custom], of not bending the knee upon Sunday, is a symbol of the resurrection, through which we have been set free, by the grace of Christ, from sins, and from death, which has been put to death under Him. Now this custom took its rise from apostolic times, as the blessed Irenaeus, the martyr and bishop of Lyons, declares in his treatise On Easter, in which he makes mention of Pentecost also; upon which [feast] we do not bend the knee, because it is of equal significance with the Lord’s day, for the reason already alleged concerning it.
from Clement of Alexandria's The Instructor:
He states that the "the Christian Passover and the weekly Lord’s Day" are one of the areas that Christians are different for Jews.
from Clement of Alexandria's Stromata:
He, in fulfillment of the precept, according to the Gospel, keeps the Lord’s day, when he abandons an evil disposition, and assumes that of the Gnostic, glorifying the Lord’s resurrection in himself.
From Tertullian:
"We count fasting or kneeling in worship on the Lord’s day to be unlawful. We rejoice in the same privilege also from Easter to Whitsunday."
"We kneel at other times, but on the Lord’s day, and from the
Paschal Feast to Pentecost we stand in prayer, nor do we count it lawful to fast on Sundays."
from Origen against Celsus:
"If it be objected to us on this subject that we ourselves are accustomed to observe certain days, as for example the Lord’s day, the Preparation, the Passover, or Pentecost, I have to answer, that to the perfect Christian, who is ever in his thoughts, words, and deeds serving his natural Lord, God the Word, all his days are the Lord’s, and he is always keeping the
Lord’s day."
from the Epistles of Cyprian:
"For because the eighth day, that
is, the first day after the Sabbath, was to be that on which the Lord should rise again, and should quicken us, and give us circumcision of the spirit, the eighth day, that is, the first day after the Sabbath, and the Lord’s day, went before in the figure; which figure ceased when by and by the truth came, and spiritual circumcision was given to us."
from the the Canonical Epistle of Peter:
"No one shall find fault with us for observing the fourth day of the week, and the preparation, on which it is reasonably enjoined us to fast according to the tradition. On the fourth day, indeed, because on it the Jews took counsel for the betrayal of the Lord; and on the sixth, because on it He himself suffered for us. But the Lord’s day we celebrate as a day of joy, because on it He rose again, on which day we have received it for a custom not even to bow the knee...But on the Lord’s day we ought not to fast, for it is a day of joy for the resurrection of the Lord, and on it, says he, we have received that we ought not even to bow the knee."
Watch out for the potholes.
