178 Take special care that as many as possible, not only of the clergy but of the laity and especially those in religious organizations and in the ranks of Catholic Action, take part in monthly days of recollection and in retreats of longer duration made with a view to growing in virtue. As We have previously stated, such spiritual exercises are most useful and even necessary to instill into souls solid virtue, and to strengthen them in sanctity so as to be able to derive from the sacred liturgy more efficacious and abundant benefits.
Source: Mediator Dei (Vatican.va)