§1. Absolution cannot be imparted in a general manner to many penitents at once without previous individual confession unless: 1/ danger of death is imminent and there is insufficient time for the priest or priests to hear the confessions of the individual penitents; 2/ there is grave necessity, that is, when in view of the number of penitents, there are not enough confessors available to hear the confessions of individuals properly within a suitable period of time in such a way that the penitents are forced to be deprived for a long while of sacramental grace or holy communion through no fault of their own. Sufficient necessity is not considered to exist when confessors cannot be present due only to the large number of penitents such as can occur on some great feast or pilgrimage.
§2. It belongs to the diocesan bishop to judge whether the conditions required according to the norm of §1, n. 2 are present. He can determine the cases of such necessity, attentive to the criteria agreed upon with the other members of the conference of bishops.
Source: Code of Canon Law (Vatican.va)