§1. After a pastor has taken possession of his parish, he is obliged to apply a Mass for the people entrusted to him on each Sunday and holy day of obligation in his diocese. If he is legitimately impeded from this celebration, however, he is to apply it on the same days through another or on other days himself.
§2. A pastor who has the care of several parishes is bound to apply only one Mass for the entire people entrusted to him on the days mentioned in §1.
§3. A pastor who has not satisfied the obligation mentioned in §§1 and 2 is to apply as soon as possible as many Masses for the people as he has omitted.
Source: Code of Canon Law (Vatican.va)