Feast of Archangles of Michael, Gabriel, and Raphael.

Saint Michael, ora pro nobis!

Feast (1969 Calendar): September 29

Angels are an essential part of the Christian faith. They are a "truth of the faith" and are mentioned over a hundred times in the Bible. We must believe in angels and fallen angels, devils, to be Christian. In honor of today's feast of the archangels, please say the full-length St. Michael prayer.

The information below is from Catholic Culture:

The liturgy celebrates the feast of these three archangels who are venerated in the tradition of the Church. Michael (Who is like God?) was the archangel who fought against Satan and all his evil angels, defending all the friends of God. He is the protector of all humanity from the snares of the devil. Gabriel (Strength of God) announced to Zachariah the forthcoming birth of John the Baptist, and to Mary, the birth of Jesus. His greeting to the Virgin, "Hail, full of grace," is one of the most familiar and frequent prayers of the Christian people. Raphael (Medicine of God) is the archangel who took care of Tobias on his journey. 

Before the reform of the General Roman Calendar today was only the feast of St. Michael. St. Gabriel was observed on March 24 and St. Raphael on October 24.
The Catechism of the Catholic Church teaches us that "[T]he existence of the spiritual, non-corporeal beings that Sacred Scripture usually calls 'angels' is a truth of faith. The witness of Scripture is as clear as the unanimity of Tradition." 

Angels are pure, created spirits. The name angel means servant or messenger of God. They are celestial or heavenly beings, on a higher order than human beings. An angel has no body and does not depend on matter for his existence or activity. They are distinct from saints, which men can become. Angels have intellect and will, and are immortal. They are a vast mulitude, but each is an individual person. Archangels are one of the nine choirs of angels listed in the Bible. In ascending order, the choirs or classes are 1) Angels, 2) Archangels, 3) Principalities, 4) Powers, 5) Virtues, 6) Dominations, 7) Thrones, 8) Cherubim and 9) Seraphim.