Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Councils of Ancyra, Gangra, Neocæsarea, Antioch and Laodicea, which Canons were Accepted and Received by the Ecumenical Synods.: Canon XVIII

Canon XVIII.

If any one, under pretence of asceticism, shall fast on Sunday, let him be anathema.

Notes.

Ancient Epitome of Canon XVIII.

Whoso fasts on the Lord’s day or on the Sabbath let him be anathema.

Zonaras.

Eustathius appointed the Lord’s day as a fast, whereas, because Christ rose from the p. 100 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.

Balsamon.

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.

This canon is found in the Corpus Juris Canonici, Gratian’s Decretum, Pars I., Dist. xxx., c. vij.