Library Eusebius of Caesarea Life of Constantine
Book 4, Chapter 23: How he forbade Idolatrous Worship, but honored Martyrs and the Church Festivals.
Such were his sacred ministrations in the service of his God. At the same time, his subjects, both civil and military, throughout the empire, found a barrier everywhere opposed against idol worship, and every kind of sacrifice forbidden. A statute was also passed, enjoining the due observance of the Lord's day, and transmitted to the governors of every province, who undertook, at the emperor's command, to respect the days commemorative of martyrs, and duly to honor the festal seasons in the churches: and all these intentions were fullfilled to the emperor's entire satisfaction.
English translation by Ernest Cushing Richardson, in Nicene and Post-Nicene Fathers, Series II, Vol. I (eds. Philip Schaff and Henry Wace; Buffalo: Christian Literature Publishing Co., 1890). Public domain. Digital text via New Advent Church Fathers.