§1. One promoted as bishop cannot assume the exercise of the office entrusted to him before he has taken canonical possession of the diocese. Nevertheless, he is able to exercise offices which he already had in the same diocese at the time of promotion, without prejudice to the prescript of ⇒ can. 409, §2.
§2. Unless he is prevented by a legitimate impediment, one promoted to the office of diocesan bishop must take canonical possession of his diocese within four months of receipt of the apostolic letter if he has not already been consecrated a bishop; if he has already been consecrated, within two months from receipt of this letter.
§3. A bishop takes canonical possession of a diocese when he personally or through a proxy has shown the apostolic letter in the same diocese to the college of consultors in the presence of the chancellor of the curia, who records the event. In newly erected dioceses, he takes canonical possession when he has seen to the communication of the same letter to the clergy and people present in the cathedral church, with the senior presbyter among those present recording the event.
§4. It is strongly recommended that the taking of canonical possession be done within a liturgical act in the cathedral church with the clergy and people gathered together.
Source: Code of Canon Law (Vatican.va)