The dogma
Catholic teaching holds that Mary was preserved by divine grace from the stain of original sin that the Church teaches affects the rest of humanity, and that she lived a life entirely free of sin. Pope Pius IX formally defined this as Church dogma in his 1854 bull Ineffabilis Deus, citing scriptural support — such as the angel Gabriel's greeting to Mary as "full of grace" — and the writings of early Church figures including Irenaeus of Lyon and Ambrose of Milan.
The feast date
The feast marking the doctrine, celebrated December 8, was declared a universal feast by Pope Sixtus IV in 1476 — nearly four centuries before the dogma itself was formally defined.