The Feast of the Immaculate Conception is a Holy Day of Obligation (opportunity).
The Holy Sacrifice of the Mass will be celebrated at:
8:00AM | 12:00PM | 5:15PM | 7:00PM Latin Mass
More on the Immaculate Conception:
In the Constitution Ineffabilis Deus of 8 December, 1854, Pius IX pronounced and defined that the Blessed Virgin Mary “in the first instance of her conception, by a singular privilege and grace granted by God, in view of the merits of Jesus Christ, the Saviour of the human race, was preserved exempt from all stain of original sin.”
To read more – click here.
Above text taken from “Immaculate Conception.” The Catholic Encyclopedia.