§1. A plenary council, that is, one for all the particular churches of the same conference of bishops, is to be celebrated whenever it seems necessary or useful to the conference of bishops, with the approval of the Apostolic See.
§2. The norm established in §1 is valid also for the celebration of a provincial council in an ecclesiastical province whose boundaries coincide with the territory of a nation.
Source: Code of Canon Law (Vatican.va)