Canon XVIII.
Canons of Local Councils Received Ecumenically (Ancyra, Gangra, Neocaesarea, Antioch, Laodicea) · The Seven Holy Ecumenical Councils
Eustathius appointed the Lord’s day as a fast, whereas, because Christ rose from the grave and delivered human nature from sin on that day, we should spend it in offering joyous thanks to God. But fasting carries with it the idea of grief and sorrow. For this reason those who fast on Sunday are subjected to the punishment of anathema.
LocalE 3.21.7
By many canons we are warned against fasting or grieving on the festal and joyous Lord’s day, in remembrance of the resurrection of the Lord; but that we should celebrate it and offer thanks to God, that we be raised from the fall of sin. But this canon smites the Eustathians with anathema because they taught that the Lord’s days should be fasted. Canon LXIV. of the Apostolic Canons cuts off such of the laity as shall so fast, and deposes such of the clergy. See also Canon LV. of the Council in Trullo.
LocalE 3.21.9