To understand today's feast day, it is necessary to know an event which took place at the time of Pope Gelasius I in the latter part of the 5th century. In Apulia, there is a mountain formerly called Gargano, now Monte Sant'Angelo or Angel's Mountain.

Near this mountain, a herdsman was keeping his cattle. A steer strayed away from his herd and went into the woods on the mountain, to the entrance of a cave which was concealed by bushes. The herdsman, to drive the animal out of the cave back to the herd, shot an arrow at it. The arrow, however, turned and flew with great force back to him. The herdsman and those who were present were terrified at this and none dared to go nearer to the cave.

They went to the Bishop, who was at Siponto, a neighbouring city, and informed him of what had occurred. The Bishop, not doubting that a Divine mystery was concealed under it, ordered his congregation to fast and pray three days, in order that God might graciously reveal it to them. At the expiration of the three days St. Michael the Archangel appeared to the Bishop and announced to him that the place whither the steer had fled was under his especial protection, and that he desired that they should dedicate the spot to the honour of God, and to the memory of St. Michael and all the Angels.

The Bishop, greatly rejoiced, called the clergy and the people together, and having informed them of the revelation, formed a large procession and ascended the mountain. They found a large cave which was like a Church hewn out of the rock. Above the entrance was an opening by which the whole interior received light. To offer the Holy Sacrifice of the Mass in it, only the Altar was wanting; but it was speedily erected by the pious Bishop. The Church itself was soon after dedicated with great solemnity to St. Michael and all Holy Angels. The fame of this event spread in a short time all around and drew a great many pilgrims to the Church, while the many miracles that took place there, were a visible sign that the veneration and invocation of St. Michael and the other Holy Angels must be very agreeable to the Most High. Today's feast day was instituted to commemorate the dedication of the Church on Mt. Gargano, hence it is called the Dedication of St. Michael, as he is especially venerated in that Church. The commemoration of the apparition of this Holy Archangel on Mt. Gargano is celebrated on 8 May.