There are several feasts of St. Michael in the Liturgy. Very ancient is the commemoration of the dedication of the Lombard sanctuary on Sant’Angelo on Monte Gargano in Apulia, near the ancient Sipontum. There is a tradition that St. Michael appeared on this mountain and requested that a church be erected to him and all the angels.
In the Old and New Testaments St. Michael, in the struggle against the Evil One, is always depicted as the invincible champion of God. The mystery of iniquity which, according to St. Paul, shall be boldly revealed in the last ages of the world and which has already begun its work of perversion, meets at present with an obstacle which hinders its full development. This obstacle is usually said to be the protection of St. Michael. St. Michael is appointed by God himself to be the protector and defender of the Church. After the protective duty conferred on St. Joseph, there is no work on earth of such great importance and sublimity as that entrusted to St. Michael.
(Source: The New Roman Missal by Father Lasance)