The feast of the Immaculate Conception of the Blessed Virgin Mary was instituted to solemnize the exalted dignity bestowed upon her through a special grace of God, and in behalf of the merits of Christ – a dignity and choice that preserved her from the least stain of original sin.
We must look to the Orient for the first indications of this feast. In the Eastern Church it was observed as early as the Seventh Century, and in the Western Churches it became known in the Ninth Century. Pope Sixtus IV introduced it (1476) into the Roman Church; Pope Gregory XV gave to it (1622) the title, “Conceptio B.V.M. Immaculatae”; Innocent XII raised it (1693) to a feast of the second class with an octave; Clement XI declared it (1708) a universal holyday; Pius IX gave it (1863) new Breviary hours and a new Mass; Leo XIII raised it (1879) to a feast of the first class with a vigil. The choice of December 8th was determined and approved solely in accordance with its tradition. The First Plenary Council of Baltimore, held in (1846), chose the Blessed Virgin Mary in her Immaculate Conception as the principal patron of the United States.
The feast of the Immaculate Conception is not to be confused with that of the conception of Christ (feast of the Annunciation of the Blessed Virgin Mary); it is the commemoration of the day on which Mary was created by God, and that in the state of grace; whereas all other human beings come into the world stained with original sin. This idea of the feast was vigorously debated by theologians till far into the Middle Ages, but was clarified more and more under the authority of the popes, so that the dogma, announced by Pope Pius IX on December 8, 1854, as divinely revealed, had already been universally accepted as such in the Seventeenth Century.
(Source: The New Roman Missal, Rev. F. X. Lasance)