The tradition of honoring the Archangel Michael on the 29th day of September dates back to the early part of the fifth century, when a basilica was dedicated in his honor on the Via Salaria in Rome. Since 1969, this day has been celebrated as a common feast honoring Michael and the Archangels Gabriel and Raphael (who had formerly been celebrated on March 24 and October 24, respectively).
These three Archangels, and indeed all the hosts of heaven, are a "great multitude of very bright living lamps; they are the vast army of heavenly spirits, shining in the blessed light and living in great beauty and adornment, because when they were created by God they did not grasp at proud exaltation but strongly persisted in divine love" (Saint Hildegard of Bingen, Scivias 1.2.1.). Honoring God with their whole beings, the Angels are the servants and messengers of God, forever looking upon the face of the Father in heaven (cf. Matthew 18:10). More than this, however, it was the Angels who first proclaimed the Good News of the Incarnation and Resurrection and they will be present at Christ's return, which they will announce.
Even as the Church joins her voice to the Angels' song of praise in her liturgies, the Angels continue to watch over us, protecting and guiding us along life's way. Saint Aloysius Gonzaga reminds us, "Even as He has granted to them abundant gifts of grace. He will grant to you grace to be able to imitate better, through their prayers, their humility, charity, and purity, so that you may be worthy to be life the Angels and rejoice with them in their longed-for beatific vision (Meditation on the Holy Angels).
A prayer for the Feast of Saints Michael, Gabriel, and Raphael +
O God, who dispose in marvelous order ministries both angelic and human, graciously grant that our life on earth may be defended by those who watch over us as they minister perpetually to you in heaven. Through our Lord Jesus Christ, your Son, who lives and reigns with you in the unity of the Holy Spirit, one God, for ever and ever.
-This reflection is taken from my book, From Season to Season: A Book of Saintly Wisdom, and the prayer is taken from The Roman Missal.