Pope Pius IX formally instituted the celebration of the Immaculate Conception when he proclaimed this dogma on December 8, 1854. Affirming the constant faith of the Church, he expressed in that definition, the exact meaning of the truth of Mary's Immaculate Conception: that she was conceived free from the stain of original sin. This feast has been celebrated in the East, and also in many places in the West, since the eighth century. This privilege of Mary is the most beautiful fruit of her Son's Redemption. Chosen as mother of the Savior, Mary received the benefits of salvation from the moment of her conception. Christ came to take away the sin of mankind; he did not allow it to contaminate Mary. The sanctity of our Blessed Mother is a model for all Christians. We seek her intercession that we may be rid of our sins and achieve sanctity.

Taken from the Handbook of Prayers edited by Fr. Jim Socias