375. Votive Masses of the mysteries of the Lord or in honor of the Blessed Virgin Mary or of the Angels or of any given Saint or of all the Saints may be said in response to the devotion of the faithful on weekdays in Ordinary Time, even if an Optional Memorial occurs. However, it is not permitted to celebrate as Votive Masses those that refer to mysteries related to events in the life of the Lord or of the Blessed Virgin Mary, with the exception of the Mass of the Immaculate Conception, since their celebration is an integral part of the course of the liturgical year.
This makes sense–it wouldn’t be good to have parishes inventing their own liturgical calendar of “various occasions.”
376. On days when there occurs an Obligatory Memorial or on a weekday of Advent up to and including December 16, of Christmas Time from January 2, and of Easter Time after the Octave of Easter, Masses for Various Needs and Occasions and Votive Masses are in principle forbidden. If, however, some real necessity or pastoral advantage calls for it, in the estimation of the rector of the church or the Priest Celebrant himself, a Mass appropriate to the same may be used in a celebration with the people.
An interesting way of dodging what is “forbidden.” But it would make sense that a real necessity might abrogate the principle. I don’t think there’s a widespread problem, however, of clergy popping in with such a Mass.
377. On weekdays in Ordinary Time when an Optional Memorial occurs or when the Office is of the weekday, it is permissible to celebrate any Mass for Various Needs and Occasions, or use any prayer for the same, but to the exclusion of Ritual Masses.
378. Particularly recommended is the Saturday commemoration of the Blessed Virgin Mary, because it is to the Mother of the Redeemer that in the Liturgy of the Church firstly and before all the Saints veneration is given.[Lumen Gentium 54; Marialis Cultus 9]