Mary the Mother of God is venerated on December 8th as the Immaculate Conception. The feast commemorates the preservation of the Blessed Virgin from the stain of original sin from the first moment of her conception. It originated in the Eastern Church about the 8th century.
The doctrine of the Immaculate Conception was defined by Pope Pius IX on December 8, 1854. This dogma, in accord with the texts of Scripture -- "I will put enmity between you and the woman, between your seed and her seed" (Gn 3:15); "Hail, full of grace" (Luke 1:28) - was clearly understood and accepted by Tradition, by the writings of the Fathers, and by feasts observed according to the general belief of the faithful long before it was officially defined by Pope Pius IX.
PRAYER God, through the Immaculate Conception of the Virgin, You prepared a worthy place for Your Son. In view of the foreseen Death of Your Son, You preserved her from all sin. Through her intercession grant that we may also reach You with clean hearts. Amen