Today we celebrate the Immaculate Conception of the Blessed Virgin Mary (a Holy Day of Obligation, don't forget). This is probably one of the most misunderstood doctrines of the Catholic Church (even among Catholics!) It does NOT refer to the conception of Jesus Christ. Rather, it refers to the conception of the Blessed Virgin in the womb of her dear mother, St. Ann. The video below gives an excellent explanation of this doctrine.
O Mary, conceived without sin, pray for us who have recourse to thee!