Constantine may have made Sunday observance an empire-wide law for the whole Roman Empire during his tenure in the 300s AD; however, we have historical writings that prove that Sunday, the Lord's Day, was observed as the public worship day by all Christian believers since Jesus rose from the dead in the first century. Jesus taught his disciples to do this. They obeyed his command!
I hope that you will be able to comprehend this difference.
The Didache, The Teachings of the Apostles, is dated to the first century AD.
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. Fort his 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."
Justin Martyr FIRST APOLOGY, circa 153-155 A.D., CHAPTER LXVII -- WEEKLY WORSHIP OF THE CHRISTIANS.
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,