December 8 is the Feast of the Immaculate Conception.
It is a Holy Day of Obligation, which means Catholics are required to attend Mass.
Today the Church celebrates the Solemnity of the Immaculate Conception, the solemn dogma defined by Blessed Pope Pius IX in 1854. As Our Lady Immaculately Conceived is the patroness of the United States of America, this is a holy day of obligation in the United States.
Through the centuries the Church has become ever more aware that Mary, "full of grace" through God, was redeemed from the moment of her conception. That is what the dogma of the Immaculate Conception confesses, as Blessed Pope Pius IX proclaimed on December 8, 1854: "The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Saviour of the human race, preserved immune from all stain of original sin." — Catechism of the Catholic Church
Today also marks the end of Celebrations for the 150th anniversary of the Apparitions of the Blessed Virgin Mary in Lourdes. From 8 December 2007 to 8 December 2008 Lourdes celebrated a Jubilee Year for the 150th anniversary of the apparitions at the Grotto of Massabielle.