21 Since priestly training, because of the circumstances particularly of contemporary society, must be pursued and perfected even after the completion of the course of studies in seminaries, it will be the responsibility of episcopal conferences in individual nations to employ suitable means to this end. Such would be pastoral institutes working together with suitably chosen parishes, meetings held at stated times, and appropriate projects whereby the younger clergy would be gradually introduced into the priestly life and apostolic activity, under its spiritual, intellectual, and pastoral aspects, and would be able, day by day, to renew and foster them more effectively.
Source: Optatam Totius (Vatican.va)