Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Council in Trullo; Often Called The Quinisext Council.: Canon VIII

Canon VIII.

Since we desire that in every point the things which have been decreed by our holy fathers may also be established and confirmed, we hereby renew the canon which orders that synods of the bishops of each province be held every year where the bishop of the metropolis shall deem best.  But since on account of the incursions of barbarians and certain other incidental causes, those who preside over the churches cannot hold synods twice a year, it seems right that by all means once a year—on account of ecclesiastical questions which are likely to arise—a synod of the aforesaid bishops should be holden in every province, between the holy feast of Easter and October, as has been said above, in the place which the Metropolitan shall have deemed most fitting.  And let such bishops as do not attend, when they are at home in their own cities and are in good health, and free from all unavoidable and necessary business, be fraternally reproved.

Notes.

Ancient Epitome of Canon VIII.

Whenever it is impossible to hold two synods a year, one at least shall be celebrated, between Easter and the month of October.

This canon under the name of the “Sixth Synod” is referred to in Canon VI. of the Seventh Ecumenical Council (II. Nice), and the bishops of Quinisext are called “Fathers.”

Van Espen.

What at first was only allowed on account of necessity, little by little passed into general law, and at last was received as law, that once a year there was to be a meeting of the provincial synod.