In the Catholic Church, the Immaculate Conception of the Blessed Virgin Mary is a holy day of obligation. Celebrated annually on December 8, it is also the Patronal Feast Day of the United States of America.
The Immaculate Conception is the belief that Mary, the mother of Jesus, was graced by God in such a way that she was preserved from sin from the first moment of her existence. “Immaculate” thus refers to the absence of sin or stain, and “Conception” refers to Mary’s very beginning. This belief has nothing to do with the sexual manner of Mary’s conception, nor with the way in which she later conceived her own child through the overshadowing of the Holy Spirit (the virginal conception of Jesus). Rather, it points to God’s graciousness to this woman as she came into the world sinless or “full of grace.”
The above paragraph is from The Modern Catholic Encyclopedia.
Click HERE to review a prayer posted on the United States Conference of Catholic Bishops (USCCB) website for the Solemnity of the Immaculate Conception of the Blessed Virgin Mary.