Sunday according to the Judeo-Christian calender is the first day of the week, now it used to be that the Sabath was the 7th day of the week but the Roman Emperor Constantine declared Sunday to be a day of reset because the people were no longer holding Saturday Sabath. Further down the line in history the Popes formalised Sunday as the Sabath but some critics say the Catholic Church does not have substantial proof of this change. They say the Catholic Chruch claims to have changed the Sabatth as part of Papal Doctrine to follow the will of God - but if Constantine had already changed the day, the Papacy is only endorsing a practice that has already been there. And instead of reverting back to the traditional Saturday Sabath, the Chruch endorsed a secular practice to become a holy one.
But then there is another conflicting side that claims an Emperor named Hadarin introduced Sunday Ises long before Constantine and the Papacy (which only came into existence abt 600AD but Hadarin reigned abt 112 - 138AD) It was an attempt by the Roman Christians to distance themselves from the Jews who were being persecuted at the time so it was began as an act of self defence and protection nothing to do with the Papacy at all. Some say the earliest record of Sunday worship was 33AD so not even Hadarin, Constantine or the Papacy have anything to do with it, but it was a practice of the apostles as seen in Acts 20:7 and 1 Cor 16:1-2. Also read Now That's Clear - Schreven (1994)
This is from www.bible.ca in support of the view Sunday Worship has always been a Christian practice:
100 AD BARNABAS "We keep the eighth day [Sunday] with joyfulness, the day also on which Jesus rose again from the dead" (The Epistle of Barnabas, 100 AD 15:6-8).
100 AD BARNABAS: Moreover God says to the Jews, 'Your new moons and Sabbaths 1 cannot endure.' You see how he says, 'The present Sabbaths are not acceptable to me, but the Sabbath which I have made in which, when I have rested [heaven: Heb 4] from all things, I will make the beginning of the eighth day which is the beginning of another world.' Wherefore we Christians keep the eighth day for joy, on which also Jesus arose from the dead and when he appeared ascended into heaven. (15:8f, The Epistle of Barnabas, 100 AD, Ante-Nicene Fathers , vol. 1, pg. 147)"
150AD JUSTIN: "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." (First apology of Justin, Weekly Worship of the Christians, Ch 68)