On this day we celebrate the Immaculate Conception of Mary the Mother of God. Her conception is referred to as "immaculate" because she was conceived without the stain of Original Sin.
"No sin would touch her, so that she would be afitting and worthy vessel of the Son of God. The Immaculate Conception does not refer to the virginal conception and the birth of Christ, but rather to Mary's being conceived without inheriting Original Sin." (United States Catholic Catechism for Adults, 142-143)