§1. The person who has been promoted to carry out the pastoral care of a parish obtains this care and is bound to exercise it from the moment of taking possession.
§2. The local ordinary or a priest delegated by him places the pastor in possession; he is to observe the method accepted by particular law or legitimate custom. The same ordinary, however, can dispense from that method for a just cause; in this case, the notification of the dispensation to the parish replaces the taking of possession.
§3. The local ordinary is to prescribe the time within which possession of a parish must be taken. When this has elapsed without action, he can declare the parish vacant unless there was a just impediment.
Source: Code of Canon Law (Vatican.va)