The Vigil of St. Matthias, the Apostle

5_14_matthiasCommemoration is made, in the Mass of St. Peter Damian, of the vigil or eve of St. Matthias, Apostle, whose feast is kept on February 24, except in leap year; at that time it is kept on February 25.
This vigil was instituted by St. Pius V in order to raise the feast of St. Matthias to the same dignity as that of the feast of the other apostles.
(Source: Fr. Lasance, The New Roman Missal)