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 XX

Canon XX.

With a view to the good of the Church and the settlement of disputes, it is decreed to be well that synods of the bishops, (of which the metropolitan shall give notice to the provincials), should be held in every province twice a year, one after the third week of the feast of Easter, so that the synod may be ended in the fourth week of the Pentecost; and the second on the ides of October which is the tenth [or fifteenth] day of the month Hyperberetæus; so that presbyters and deacons, and all who think themselves unjustly dealt with, may resort to these synods and obtain the judgment of the synod.  But it shall be unlawful for any to hold synods by themselves without those who are entrusted with the Metropolitan Sees.

Notes.

Ancient Epitome of Canon XX.

On account of ecclesiastical necessities the synod in every province shall meet twice a year, in the fourth week of Pentecost and on the tenth day of Hyperberetæus.

Schelestratius (cit. Van Espen).

The time fixed by the Council of Nice before Lent for the meeting of the synod was not received in the East, and the bishops kept on in the old custom of celebrating the council in the fourth week after Easter, for the time before Lent often presented the greatest difficulties for those in the far separated cities to come to the provincial metropolis.

Van Espen.

In this canon the decree of Nice in canon v. is renewed, but with this difference that the Nicene synod orders one synod to be held before Lent, but this synod that it should be held the fourth week after Easter.

It will be remembered that the whole period of the great fifty days from Easter to Whitsunday was known as “Pentecost.”

Compare with this Apostolic Canon number XXXVII.

This canon is found in the Corpus Juris Canonici, Gratian’s Decretum, Pars I., Dist. XVIII., c. xv., attributed to a council held by Pope Martin.  The Roman Correctors point out that this “Pope Martin” was a bishop of Braga (Bracarensis) from whose collection of the decrees of the Greek synods Gratian often quotes; the Correctors also note, “For bishops in old times were usually called Popes” (Antiquitus enim episcopi Papæ dicebantur).