The day preceding a festival is styled a vigil (from the Latin word signifying a night-watch) because in primitive ages the faithful passed in prayer in the church the greater part of the evening and night preceding a festival. Nor did they break their fast until after the holy sacrifice of the Mass had been offered, and Communion given in the course of the vigil. Hence the greater vigils are still observed as fast days; and the Mass of a vigil has a special penitential character. Purple vestments are worn by the priest; the Gloria in excelsis is not said.

St. Saturninus, martyr, suffered at Rome in the last great persecution under Emperor Diocletian, 304.

(Source: Fr. Lasance, The New Roman Missal,)