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 XXXIX

Canon XXXIX.

It is not lawful to feast together with the heathen, and to be partakers of their godlessness.

Notes.

Ancient Epitome of Canons XXXVII., XXXVIII, and XXXIX.

Thou shalt not keep feasts with Hebrews or heretics, nor receive festival offerings from them.

Balsamon.

Read canon lxx. and canon lxxj. of the Holy Apostles, and Canon lx. 197 of the Synod of Carthage.

Aristenus.

Light hath no communion with darkness.  Therefore no Christian should celebrate a feast with heretics or Jews, neither should he receive anything connected with these feasts such as azymes and the like.

Footnotes

So both Zonaras and Balsamon give the number, but in this they follow the Latin numbers of the African Code, the Greek number is lxiij.