This is the original feast of the leader of the heavenly armies, St. Michael. The captain of the heavenly armies, the angel named in the Canon of the Mass, held from early times the first place in the Liturgy among the other angels; wherefore many churches dedicated to St. Michael in the Middle Ages were simply known as churches “of the holy angel.”
St. Michael, whose name signifies “who is like unto God” cast the evil spirit out of heaven, and overcame Satan in the struggle for possession of the body of Moses. God has entrusted our defense, in the combat with the devil, to the angels. The reason of this is easily understood. The devil is a spirit who has lost none of the powers inherent to his nature. In order, therefore, that the struggle should not be unequal, God has placed at our side defenders of the same nature as Lucifer, that is to say pure spirits, who are, however, greater and more powerful than he is.
(Source: The New Roman Missal, Rev. F. X. Lasance)