Edict of Constantine
On 3 March 321, Constantine I decreed that Sunday (dies Solis) will be observed as the Roman day of rest [CJ3.12.2]:
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 suitable for grain-sowing or 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. Codex Justinianus, lib. 3, tit. 12, 3; translated by Philip Schaff, History of the Christian Church, Vol. 3 (1902), p. 380, note.)
Constantine’s decree was most likely modeled on pagan sun worship, though it is probable that he also intended to benefit the church, which already met for worship on Sunday. Some theorize that, because the practice favored the Christian day of worship, it also helped the church to avoid implicit association with the Jews. Eusebius, in Life of Constantine, claims Constantine stated: “Let us then have nothing in common with the detestable Jewish crowd; for we have received from our Saviour a different way.” (Book III, Chapter 18, newadvent.org/fathers/25023.htm)
Chapter 18. He speaks of their Unanimity respecting the Feast of Easter, and against the Practice of the Jews.
At this meeting the question concerning the most holy day of Easter was discussed, and it was resolved by the united judgment of all present, that this feast ought to be kept by all and in every place on one and the same day. For what can be more becoming or honorable to us than that this feast from which we date our hopes of immortality, should be observed unfailingly by all alike, according to one ascertained order and arrangement? And first of all, it appeared an unworthy thing that in the celebration of this most holy feast we should follow the practice of the Jews, who have impiously defiled their hands with enormous sin, and are, therefore, deservedly afflicted with blindness of soul. For we have it in our power, if we abandon their custom, to prolong the due observance of this ordinance to future ages, by a truer order, which we have preserved from the very day of the passion until the present time. Let us then have nothing in common with the detestable Jewish crowd; for we have received from our Saviour a different way. A course at once legitimate and honorable lies open to our most holy religion. Beloved brethren, let us with one consent adopt this course, and withdraw ourselves from all participation in their baseness. For their boast is absurd indeed, that it is not in our power without instruction from them to observe these things. For how should they be capable of forming a sound judgment, who, since their parricidal guilt in slaying their Lord, have been subject to the direction, not of reason, but of ungoverned passion, and are swayed by every impulse of the mad spirit that is in them? Hence it is that on this point as well as others they have no perception of the truth, so that, being altogether ignorant of the true adjustment of this question, they sometimes celebrate Easter twice in the same year. Why then should we follow those who are confessedly in grievous error? Surely we shall never consent to keep this feast a second time in the same year. But supposing these reasons were not of sufficient weight, still it would be incumbent on your Sagacities to strive and pray continually that the purity of your souls may not seem in anything to be sullied by fellowship with the customs of these most wicked men. We must consider, too, that a discordant judgment in a case of such importance, and respecting such religious festival, is wrong. For our Saviour has left us one feast in commemoration of the day of our deliverance, I mean the day of his most holy passion; and he has willed that his Catholic Church should be one, the members of which, however scattered in many and diverse places, are yet cherished by one pervading spirit, that is, by the will of God. And let your Holinesses’ sagacity reflect how grievous and scandalous it is that on the self-same days some should be engaged in fasting, others in festive enjoyment; and again, that after the days of Easter some should be present at banquets and amusements, while others are fulfilling the appointed fasts. It is, then, plainly the will of Divine Providence (as I suppose you all clearly see), that this usage should receive fitting correction, and be reduced to one uniform rule.
Life of Constantine, Eusebius (Book III)
Fathers of the Church