Here are some interesting historical references you might find interesting that clear up a few misconception people may have about Sunday worship entering the Roman empire and the early Church....
Historically, Constantine did not change the day of worship as many claim. According to the historical records, Constantine made a civil Sunday law that the Roman Catholic Church adopted in order to practice and promote Sunday worship over the Sabbath of God's 4th commandment...
First Sunday Law enacted by Emperor Constantine - March, 321 A.D.
“On the venerable Day of the Sun let the magistrates and people residing in cities rest, and let all workshops be closed. In the country, however, persons engaged in agriculture may freely and lawfully continue their pursuits; because it often happens that another day is not so suitable for grain-sowing or for vine-planting; lest by neglecting the proper moment for such operations the bounty of heaven should be lost. (Given the 7th day of March, Crispus and Constantine being consuls each of them for the second time [A.D. 321].)” Source: Codex Justinianus, lib. 3, tit. 12, 3; trans. in Philip Schaff, History of the Christian Church, Vol.3 (5th ed.; New York: Scribner, 1902), p.380, note 1.
Now a professed Christian, Constantine nevertheless remained a devout sun worshipper. “The sun was universally celebrated as the invincible guide and protector of Constantine,” notes Edward Gibbon in his classic Decline and Fall of the Roman Empire, ch. xx, par. 3.
Constantine even printed coins which “bore on the one side the letters of the name of Christ, on the other the figure of the sun god.” Arthur P. Stanley, History of the Eastern Church, lect. vi, par. 14.
Again, Constantine’s promotion of Sunday observance was part of his definite strategy to combine paganism with Christianity: “The retention of the old pagan name of dies Solis, or 'Sunday,' for the weekly Christian festival, is in great measure owing to the union of pagan and Christian sentiment with which the first day of the week was recommended by Constantine to his subjects, pagan and Christian alike, as the ‘venerable day of the Sun.’” – Stanley’s History of the Eastern Church, p. 184.
In spite of the rising popularity of Sunday sacredness, Church historian Socrates Scholasticus (5th century) wrote: “For although almost all churches throughout the world celebrate the sacred mysteries [of the Lord's Supper] on the Sabbath of every week, yet the Christians of Alexandria and at Rome, on account of some ancient tradition, have ceased to do this.” – Socrates Scholasticus, Ecclesiastical History, Book 5, ch. 22.
Another historian also confirmed this by stating, “The people of Constantinople, and almost everywhere, assemble together on the Sabbath, as well as on the first day of the week, which custom is never observed at Rome or at Alexandria.” – Sozomen, Ecclesiastical History, Book 7, ch. 19. Thus even in the 5th century, Sabbath keeping was universally prevalent (except in Rome and Alexandria) along with Sunday keeping. Many Christians kept both days, but as the centuries wore on, Sunday keeping grew in prominence and especially within Roman Catholic territories.
In 330 A.D., Constantine moved his capital from Rome to Constantinople (modern Istanbul), thus preparing the way for the Roman Catholic Popes to reign in Rome as the successors of Constantine. As the Papal Church grew in power, it opposed Sabbath observance in favour of Sunday sacredness and made the day change official in the Council of Laodicea (A.D. 363-364). Constantine's law had now been fully integrated into the Papal Church and the final step of the Sabbath to Sunday change was complete.
So around the year A.D. 364, the Catholic Church outlawed Sabbath keeping in the Council of Laodicea when they decreed 59 Canon laws. The following is the relevant Canon law: Canon XXIX: “Christians must not judaize by resting on the Sabbath, but must work on that day, rather honouring the Lord's Day; and, if they can, resting then as Christians. But if any shall be found to be judaizers, let them be anathema from Christ.” (Percival Translation).
Four hundred years after the death of Christ and one hundred years after Constantine's linking of Church and State by his Sunday law edict, Rome and Alexandria were the only places in the world where many of the Christians kept only Sunday and not the true Sabbath. Why was it that Rome and Alexandria were also the first locations that Sunday worship began? Because this is where the pagan practices of Babylon eventually landed after it was conquered. And what was the dominant pagan practice that the Babylonian priests brought with them? Sun worship which was done on Sun-day! See the history of Sunday worship for more detail.
So one can understand why Rome and Alexandria did not bother to keep the true Sabbath as they had not done so for 200 years. Throughout the entire history of the changeover from Sabbath to Sunday, Rome and Alexandria had worked together. Alexandria provided the philosophical reasons for the changes and Rome provided the decrees and anathemas. Constantine's help was given only to the worldly Church leaders at Rome and those Christians that resisted the errors that were being introduced into the Church met with his opposition. “Unite with the bishop of Rome or be destroyed,” was Constantine's position.
“Great as were the favors which Constantine showed to the church, they were only for that strong, close-knit, hierarchically organized portion that called itself Catholic. The various [so-called] heretical sects could look for no bounty from his hands.” – Williston Walker, A History of the Christian Church, page 105.
The change of the Sabbath to Sunday was totally completed by the seventh century as the Popes consolidating their enormous power persecuted all who resisted their innovations. Did Satan use Constantine to play a key part in his plan to change the Sabbath to his day being Sunday? The answer is clear! From sun worship 2000 B.C., to Sunday worship in the Church. Satan infiltrated the Christian Church and most are oblivious to the fact that this has happened or understand the relevance. Sunday or “dies solis”, the day of the sun came from Satan worship and is his day. (more from here).