The Council of Chalcedon (451 AD)

Whereas it has come to our ears that in the provinces the Canonical Synods of Bishops are not held, and that on this account many ecclesiastical matters which need reformation are neglected; therefore, according to the canons of the holy Fathers, the holy Synod decrees that the bishops of every province shall twice in the year assemble together where the bishop of the Metropolis shall approve, and shall then settle whatever matters may have arisen. And bishops, who do not attend, but remain in their own cities, though they are in good health and free from any unavoidable and necessary business, shall receive a brotherly admonition.

Ancient Epitome of Canon XIX.

Twice each year the Synod shall be held whereever the bishop of the Metropolis shall designate, and all matters of pressing interest shall be determined.

See notes on Canon V. of Nice, and on Canon XX. of Antioch, and compare canon VIII. of the council in Trullo.

Hilary of Arles and his suffragans, assembled at Riez, had already, in 439 qualified the provision for two by adding significantly “if the times are quiet”. The words were written at the close of ten years’ war, during which the Visigoths of Septimania “were endeavouring to take Arles and Narbonne” (Hodgkin, Italy and her Invaders, ii., 121).

This canon is found in the Corpus Juris Canonici, Gratian’s Decretum, Pars I., Dist. XVIII., canon vj.