The Immaculate Conception is a Catholic dogma that states that Mary, whose conception was brought about the normal way, was conceived without original sin or its stain whose essence consists in the deprivation of sanctifying grace. Mary was preserved from these defects by God’s grace; from the first instant of her existence she was in the state of sanctifying grace and was free from the corrupt nature original sin brings. An implicit biblical reference may be found in the angel Gabriel’s greeting to Mary: “Hail, full of grace, the Lord is with you” (Luke 1:28). The grace Mary enjoyed was not a result of the angel’s visit. Rather, Catholic teaching holds that it extended over the whole of her life from conception onward. 

The Solemnity of the Immaculate Conception of the Blessed Virgin Mary (December 8) is a holy day of obligation to be observed by all Latin Rite Catholics. (Canons 1246-1248)