What are votive masses, votive prayers (there’s a group of them in my prayer book, and votive candles?.
You can read more about Votive masses from newadvent.org
A Mass offered for a votum, a special intention. So we frequently find in prayers the expression, votiva dona (e.g., in the Leonine Sacramentary, ed. Feltoe, p. 103), meaning “gifts offered with desire [of receiving grace in return]”. The Mass does not correspond to the Divine Office for the day on which it is celebrated. Every day in the year has appointed to it a series of canonical hours and (except Good Friday) a Mass corresponding, containing, for instance, the same Collect and the same Gospel. So Mass and Office together make up one whole. Normally the Mass corresponds to the Office. But there are occasions on which a Mass may be said which does not so correspond. These are votive Masses.
The principle of the votive Mass is older than its name. Almost at the very origin of the Western liturgies (with their principle of change according to the Calendar) Mass was occasionally offered, apparently with special prayers and lessons, for some particular intention, irrespective of the normal Office of the day. Among the miracles quoted by St. Augustine in “De civ. Dei”, XXII, 8, is the story of one Hesperius cured of an evil spirit by a private Mass said in his house with special prayers for him – a votive Mass for his cure. The first Sacramentaries contain many examples of what we should call votive Masses. So the Leonine book has Masses “in natale episcoporum” (ed. Feltoe, pp. 123-26), “de siccitate temporis” (ibid., 142), “contra impetitores” (ibid., 27), and so on throughout. Indeed the Masses for ordination and for the dead, which occur in this book and throughout the Roman and Gallican Rites, are really examples of votive Masses for all kinds of occasions, for ordinations (ed. Wilson, pp. 22-30, etc.), for those about to be baptized (ibid., 34), anniversaries of ordinations (153-54), nuns (156), for the sick (282), for marriages (265), kings (276), travellers (283), the dead (301 sq.), and a large collection of Masses of general character to be said on any Sunday (224-44). In this book the name first occurs, “Missa votiva in sanctorum commemoratione” (p. 367; Rheinau and S. Gallen MSS.). The Gregorian Sacramentary, too, has a large collection of such Masses and the name “Missa votiva” (e.g., P.L., LXXVIII, 256).