It is for the conference of bishops: 1/ to convoke a plenary council; 2/ to select the place to celebrate the council within the territory of the conference of bishops; 3/ to select from among the diocesan bishops a president of the plenary council whom the Apostolic See must approve; 4/ to determine the agenda and questions to be treated, set the opening and duration of a plenary council, transfer, extend, and dissolve it.
Source: Code of Canon Law (Vatican.va)