§1. After the diocesan bishop has taken possession of the diocese, he must apply a Mass for the people entrusted to him each Sunday and on the other holy days of obligation in his region.
§2. The bishop himself must personally celebrate and apply a Mass for the people on the days mentioned in §1. If he is legitimately impeded from this celebration, however, he is to apply the Masses either on the same days through another or on other days himself.
§3. A bishop to whom other dioceses besides his own have been entrusted, even under title of administration, satisfies the obligation by applying one Mass for all the people entrusted to him.
§4. A bishop who has not satisfied the obligation mentioned in §§1-3 is to apply as soon as possible as many Masses for the people as he has omitted.
Source: Code of Canon Law (Vatican.va)