Today, December 8th, is the Solemnity of the Immaculate Conception.
Since this Feast day falls on a Sunday (and today is the Second Sunday of Advent), the celebration is transferred to Monday, December 9. The transfer is made as the Second Sunday of Advent takes precedence. In the United States, the obligation to attend Mass tomorrow is dispensed (it's typically a Holy Day of Obligation) – but do try to go to Mass tomorrow anyway to celebrate this beautiful feast day!
The Catholic Church teaches as dogma that Mary, "in the very instance of her conception by a singular privilege and grace granted by God, in view of the merits of Jesus, the Savior of the human race, was preserved from all stain of Original Sin." (Pope Pius IX)
Because of His foreknowledge that Mary would consent to bear Christ, God kept Mary free from original sin from the moment of her conception in the womb of her mother, Saint Anne.
"I am the Immaculate Conception", said Our Lady of Lourdes to St. Bernadette in an Apparition
O Mary, Conceived without sin, Pray for us who have recourse to Thee!