In the prayer for the feast of St. Michael (September 29) we ask that the Holy Angels may fulfill the special office entrusted to them of watching faithfully over us upon earth. This feast included together with St. Michael the whole of the heavenly court. A distinct feast of the Guardian Angels originated only at a later date. The Spaniards observed it in the Sixteenth Century and Paul V (September 27, 1608) declared the feast of the Guardian Angels, a feast of the whole Church, and ordered that it should be kept on the first vacant date after the feast of St. Michael. Clement X (1670) assigned it to October 2nd. Leo XIII raised it to the rank of a greater double.
The guardianship of the faithful is given by God into the hands of the angels, not only for the sake of the unity and harmony which bind together the orders of creation, but also on account of the duty owed by the angels to Christ. It is from Christ that they receive their glory, therefore out of gratitude to Him who is caput hominum et angelorum (the head of men and angels) these blessed spirits watch over the Church, the chosen Bride of the Savior, and over the faithful who are members of His Mystical Body.
(Source: The New Roman Missal, Rev. F. X. Lasance)