It has to do with the conception of Mary:
The Immaculate Conception, according to the teaching of the Catholic Church, is the conception of the Blessed Virgin Mary free from original sin by virtue of the foreseen merits of her son Jesus Christ. The Catholic Church teaches that Mary was conceived by normal biological means in the womb of her mother, Saint Anne, but God acted upon her soul, keeping it "immaculate".
The Immaculate Conception is commonly confused with the Virgin Birth of Jesus. Jesus's birth is covered by the Doctrine of Incarnation, while the Immaculate Conception deals with the conception of Mary, not that of her son.
Although the belief that Mary was sinless, or conceived with an immaculate soul, has been widely held since Late Antiquity, the doctrine was not dogmatically defined until 1854, by Pope Pius IX in his papal bull Ineffabilis Deus.[1] The Catholic Church celebrates the Feast of the Immaculate Conception on December 8; in many Catholic countries, it is a holy day of obligation or patronal feast, and in some a national public holiday.[2] - Wikipedia