Life of Constantine

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. 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), he freely granted to those 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.