Nicene and Post-Nicene Fathers, Ser. II, Vol I:
The Life of Constantine with Orations of Constantine and Eusebius.: Chapter XVIII

Chapter XVIII.—He enjoins the General Observance of the Lord’s Day, and the Day of Preparation.

He ordained, too, that one day should be regarded as a special occasion for prayer: I mean that which is truly the first and chief of all, the day of our Lord and Saviour. The entire care of his household was entrusted to deacons and other ministers consecrated to the service of God, and distinguished by gravity of life and every other virtue: while his trusty body guard, strong in affection and fidelity to his person, found in their emperor an instructor in the practice of piety, and like him held the Lord’s salutary day in honor and performed on that day the devotions which he loved. The same observance was recommended by this blessed prince to all classes of his subjects: his earnest desire being gradually to lead all mankind to the worship of God. Accordingly he enjoined on all the subjects of the Roman empire to observe the Lord’s day, as a day of rest, and also to honor the day which precedes the Sabbath; in memory, I suppose, of what the Saviour of mankind is recorded to have achieved on that day. 3318 And since his desire was to teach his whole army zealously to honor the Saviour’s day (which derives its name from light, and from the sun), 3319 he freely granted to those p. 545 among them who were partakers of the divine faith, leisure for attendance on the services of the Church of God, in order that they might be able, without impediment, to perform their religious worship.

Footnotes

[That is, Friday. The passage is not very intelligible. Does it mean that Constantine ordered this day to be distinguished in some way from others, as the day of the Lord’s crucifixion?—Bag.]

[The decree of Constantine for the general observance of Sunday appears to have been issued a.d. 321, before which time both “the old and new sabbath” were observed by Christians.

“Constantine (says Gibbon, ch. 20, note 8) styles the Lord’s day Dies solis, a name which could not offend the ears of his Pagan subjects.”—Bag.] This has been urged as ground for saying that Constantine did not commit himself to Christianity until the end of life, but it only shows his tact and care in treating the diverse elements of his empire.