57 1. Concelebration, whereby the unity of the priesthood is appropriately manifested, has remained in use to this day in the Church both in the east and in the west. For this reason it has seemed good to the Council to extend permission for concelebration to the following cases: 1. a) on the Thursday of the Lord's Supper, not only at the Mass of the Chrism, but also at the evening Mass. b) at Masses during councils, bishops' conferences, and synods; c) at the Mass for the blessing of an abbot. 2. Also, with permission of the ordinary, to whom it belongs to decide whether concelebration is opportune: a) at conventual Mass, and at the principle Mass in churches when the needs of the faithful do not require that all priests available should celebrate individually; b) at Masses celebrated at any kind of priests' meetings, whether the priests be secular clergy or religious. 2. 1. The regulation, however, of the discipline of con-celebration in the diocese pertains to the bishop. 2. Nevertheless, each priest shall always retain his right to celebrate Mass individually, though not at the same time in the same church as a concelebrated Mass, nor on Thursday of the Lord's Supper.
Source: Sacrosanctum Concilium (Vatican.va)