Feast Of The Dedication Of Saint Michael The Archangel - 29 September

Quoted from ewtn.com [1]:

The three archangels whom the Church honors by name are St. Michael, St. Gabriel, and St. Raphael. They are also the only three angels who are mentioned by name in Sacred Scripture. The Feast of all three Archangels is September 29. In the traditional calendar this was St. Michael’s feast day, with St. Gabriel’s being March 24th (the day prior to the Annunciation) and St. Raphael’s being October 24th. Communities which celebrate the Extraordinary Form of the Roman Rite still celebrate the three feasts.

Quoted from catholicharboroffaithandmorals.com [2]:

...To understand this [dedication], it is necessary to know an event which took place at the time of Pope Gelasius I. in the latter part of the fifth Century. In Apulia, there is a mountain formerly called Gargano, now Monte St. 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 neighboring 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 honor 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 festival was instituted to commemorate the dedication of the Church on Mount 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 Mount Gargano is celebrated on May 8th.

Quoted from dailyrosaryfamily.com [3]:

While St. Michaels’ [sic] Feast Day is September 29, the feast of the Apparition of Saint Michael on May 8 commemorates the 6th century appearance of the archangel on Mount Gargano near Manfredonia in southern Italy. A church was built in his honor at the site. 

[1] https://www.ewtn.com/catholicism/seasons-and-feast-days/archangels-14635

[2] http://www.catholicharboroffaithandmorals.com/Dedication%20of%20St.%20Michael.html#:~:text=Dedication%20of%20St.%20Michael%20the%20Archangel.%20On%20Mount,altar%20which%20St.%20John%20saw%20%28Apoc.%208%2C%203-4%29.

[3] http://dailyrosaryfamily.com/may-8-feast-of-the-apparition-of-st-michael-victory-gargano/#:~:text=May%208%20Feast%20of%20the%20Apparition%20of%20St.,was%20built%20in%20his%20honor%20at%20the%20site.