St. Eusebius was a Roman priest distinguished for his zeal in the cause of orthodoxy. He lived in the Fourth Century of our era. He appears to have died in prison. The universal veneration in which he was held led to his immediate canonization.
(Source: The New Roman Missal, Rev. F. X. Lasance)
As early as the Seventh Century the preparation, by solemn fast, for keeping devoutly the great festival of the Assumption, is described as a custom of great antiquity. In many parts of the Church the strict fast enjoined lasted over several days. The Mass celebrated on this day is of Our Blessed Lady, on account of its being that of a vigil, and is said in purple vestments, and the hymn Gloria in excelsis is omitted.