An apparition which
encouraged devotion to St. Michael in Italy,
took place on Mount Gargano, in Apulia;
it is the one honored by today's Feast.
A celebrated apparition of the Archangel took place, under the Pontificate of Gelasius I, in Apulia, on the top of Mount Gargano, at whose foot lies the town of Siponto.
A bull belonging to a man who lived on the mountain, having strayed from the herd, was, after much searching, found hemmed fast in the mouth of a cave. One of its pursuers shot an arrow, with a view to rouse the animal by a wound; but the arrow rebounding struck him that sent it. This circumstance excited so much fear in the bystanders and in them who heard of it, that no one dared to go near the cave. The inhabitants of Siponto, therefore, consulted the Bishop; he answered that in order to know God's will, they must spend three days in fasting and prayer.
At the end of the three days, the Archangel Michael warned the Bishop that the place was under his protection, and that what had occurred was an indication of his will that God should be worshiped there, in honor of himself and the Angels. Whereupon the Bishop repaired to the cave, together with his people. They found it like a church in shape, and began to use it for the celebration of the divine offices (see image below). Many miracles were afterward wrought there. Not long after, Pope Boniface dedicated a church in honor of St. Michael in the great Circus of Rome, on the third of the Kalends of October (September 29), the day on which the Church celebrates the memory of all the Angels. But today's Feast is kept in commemoration of the Apparition of St. Michael the Archangel. - Source
Michael, the Angel of Fatima
Some believe the Angel of Fatima was St. Michael Archangel, since he is the Guardian Angel of Portugal, and is commemorated in the liturgy as the Angel of Peace, he is also considered to be the defender of the Eucharist. All three attributes were revealed by the Angel of Fatima.
King Alfonso Henriques, founder of the nation and of the dynasty, was baptised in a chapel dedicated to St. Michael and chose the Archangel as protector of his armies and realm. Since then, the Chapel of the royal Palace has always been dedicated to him. The national devotion to the Archangel was such that, in 1514 Manuel I asked Pope Leo X to grant his nation a Feast for “ the Guardian Angel of Portugal ”. It was celebrated in July. At the national monastery of Batalha, the Portuguese Saint-Denis, where the Kings of the Avis dynasty are buried, St. Michael, guardian angel of Portugal, was prayed daily. - Read more here.