First Council of Nicaea, Canon V

Concerning those, whether of the clergy or of the laity, who have been excommunicated in the several provinces, let the provision of the canon be observed by the bishops which provides that persons cast out by some be not readmitted by others.  Nevertheless, inquiry should be made whether they have been excommunicated through captiousness, or contentiousness, or any such like ungracious disposition in the bishop.  And, that this matter may have due investigation, it is decreed that in every province synods shall be held twice a year, in order that when all the bishops of the province are assembled together, such questions may by them be thoroughly examined, that so those who have confessedly offended against their bishop, may be seen by all to be for just cause excommunicated, until it shall seem fit to a general meeting of the bishops to pronounce a milder sentence upon them.  And let these synods be held, the one before Lent, (that the pure Gift may be offered to God after all bitterness has been put away), and let the second be held about autumn.

Translation: Henry R. Percival, The Seven Ecumenical Councils, NPNF II.14 (1900), via CCEL.