Today is the Solemnity of the Immaculate Conception, which celebrates one of the great (and oftentimes misunderstood) mysteries of our faith, namely, that the Virgin Mary, from the moment of her conception, was preserved immaculate from all stain of original sin. This Marian dogma was infallibly defined by Pope Pius IX in 1854:
We declare, pronounce and define that the doctrine which holds that the Blessed Virgin Mary, at the first instant of her conception, by a singular privilege and grace of the Omnipotent God, in virtue of the merits of Jesus Christ, the Saviour of mankind, was preserved immaculate from all stain of original sin, has been revealed by God, and therefore should firmly and constantly be believed by all the faithful.”
– Pope Pius IX, Ineffabilis Deus
Here are a few informative videos clips which discuss the Immaculate Conception. Enjoy!