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My mistake. I thought it was in my book Irenaus Against Heretics. But it's in Justin Martyr's First Apology:

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,147 and the people assent, saying Amen; and there is a distribution to each, and a participation of that over which thanks have been given,148 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.

My mistake. I thought it was in my book Irenaus Against Heretics. But it's in Justin Martyr's First Apology

Oh, the good antijewish Martyr. Lets just see what he really though the reasoning behind the sabbath was....

"For since you have read, O Trypho, as you yourself admitted, the doctrines taught by our Saviour, I do not think that I have done foolishly in adding some short utterances of His to the prophetic statements. Wash therefore, and be now clean, and put away iniquity from your souls, as God bids you be washed in this laver, and be circumcised with the true circumcision. For we too would observe the fleshly circumcision, and the Sabbaths, and in short all the feasts, if we did not know for what reason they were enjoined you,-namely, on account of your transgressions and the hardness of your hearts.---Dialogue of Justin

I have a hard time believing him that this is the reason for the law. Justin here is saying that the law was giving because of the Jews sin. John said the sin is the transgression of the law(1 jn 3:4)

My mistake. I thought it was in my book Irenaus Against Heretics. But it's in Justin Martyr's First Apology:

No problem. There is no doubt that gentile Christians, which Justin Martyr was, had already begun to distance themselves from Jewish converts as soon as 100 years after the death of Christ. Martyr spent most of his life in Rome where false Gods were rampant. Perhaps worshipping on Sunday served two purposes: It didn't look so suspicious since there was weekly feast to the Sun god on that day...and it also served to make them appear less "Jewish". Either way it did seem to be a departure from from scripture.

But I think the authors intent wasn't to prove that Constantine was the first person to come up with the concept of Sunday worship, but was the person who made it official for the Roman church, which ultimately became the Roman Catholic church and her protestent sisters.