History reveals what happened...
"Unquestionably the first law, either ecclesiastical or civil, by which the Sabbatical observance of that day is known to have been ordained, is the edict of Constantine, 321 A.D."--"Chamber's Encyclopedia," article, "Sabbath."
Here is the first Sunday Law in history, a legal enactment by Constantine 1 (reigned 306-331): "On the Venerable Day of the Sun ["venerabili die Solis"--the sacred 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, [A.D. 321], Crispus and Constantine being consuls each of them for the second time."--The First Sunday Law of Constantine 1, in "Codex Justinianus," lib. 3, tit. 12, 3; trans. in Phillip Schaff "History of the Christian Church," Vol. 3, p. 380.
"This [Constantine's Sunday decree of March, 321] is the 'parent' Sunday law making it a day of rest and release from labor. For from that time to the present there have been decrees about the observance of Sunday which have profoundly influenced European and American society. When the Church became a part of State under the Christian emperors, Sunday observance was enforced by civil statutes, and later when the Empire was past, the Church, in the hands of the papacy, enforced it by ecclesiastical and also by civil enactments."--Walter W. Hyde, "Paganism to Christianity in the Roman Empire," 1946, p. 261.
"Constantine's decree marked the beginning of a long, though intermittent series of imperial decrees in support of Sunday rest."-- Vincent J. Kelly, "Forbidden Sunday and Feast-Day Occupations," 1943, p. 29.
"Constantine labored at this time untiringly to unite the worshipers of the old and the new into one religion. All his laws and contrivances are aimed at promoting this amalgamation of religions. He would by all lawful and peaceable means melt together a purified heathenism and a moderated Christianity . . . Of all his blending and melting together of Christianity and heathenism, none is more easy to see through than this making of his Sunday law: The Christians worshiped their Christ, the heathen their Sun-god . . . [so they should now be combined."--H.G. Heggtveit, "illustreret Kirkehistorie," 1895, p. 202.
"If every Sunday is to be observed joyfully by the Christians on account of the resurrection, then every Sabbath on account of the burial is to be regarded in execration [cursing] of the Jews."--Pope Sylvester, quoted by S.R.E. Humbert, "Adversus Graecorum Calumnias," in J.P. Migne, "Patrologie," p. 143. [Sylvester (A.D. 314-337) was the pope at the time Constantine 1 was Emperor.]
"All things whatsoever that were prescribed for the [Bible] Sabbath, we have transferred them to the Lord's day, as being more authoritative and more highly regarded and first in rank, and more honorable than the Jewish Sabbath."--Bishop Eusebius, quoted in J.P. Migne, "Patrologie," p. 23, 1169-1172. [Eusebius of Caesarea was a high-ranking Catholic leader during Constantine's lifetime.]
"These Gentile Christians of Rome and Alexandria began calling the first day of the week 'the Lord's day.' This was not difficult for the pagans of the Roman Empire who were steeped in sun worship to accept, because they [the pagans] referred to their sun-god as their 'Lord.' "--EM. Chalmers, "How Sunday Came Into the Christian Church," p. 3.
The following statement was made 100 years after Constantine's Sunday Law was passed: "Although almost all churches throughout the world celebrate the sacred mysteries on the Sabbath every week, yet the Christians of Alexandria and at Rome, on account of some ancient tradition, have ceased to do this."--Socrates Scholasticus, quoted in "Ecclesiastical History," Book 5, chap. 22. [Written shortly after A.D. 439.]
"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."--Hermias Sozomen, quoted in "Ecclesiastical History," vii, 19, in "A Select Library of Nicene and Post-Nicene Fathers," 2nd Series, Vol. 2, p. 390. [Written soon after AD. 415.]
"Down even to the fifth century the observance of the Jewish Sabbath was continued in the Christian church, but with a rigor and solemnity gradually diminishing until it was wholly discontinued."--Lyman Coleman, "Ancient Christianity Exemplified" chap. 26, sec. 2, p. 527.
"Constantine's [five Sunday Law] decrees marked the beginning of a long though intermittent series of imperial decrees in support of Sunday rest."--"A History of the Councils of the Church," Vol. 2, p. 316.
"What began, however, as a pagan ordinance, ended as a Christian regulation; and a long series of imperial decrees, during the fourth, fifth, and sixth centuries, enjoined with increasing stringency abstinence from labor on Sunday."--Huttan Webster, "Rest Days," pp. 122-123, 210.