God and His graces are not limited like ourselves to time and space. The merits of our Lord on the Cross are the real reason that the oxen and doves offered to God at the Temple of Jerusalem where acceptable. The offerings of the Old Testament where done based on the promise of Christ. The suffering of our Lord on the Cross was applied to those holy men and women of the Old Testament too.
So too with our Lady. An apocryphal tale of one of the Early Church Fathers, (and therefore not required belief) states that before the creation of the world, God knowing that man would fall into sin and need a savior looked out on all the souls that would ever come into exsistance. Most every soul was paying attention to themselves, but one soul seemed to be missing. On closer inspection God saw that Mary was bowed down in complete and total submission to His will. It was there that God decided that she would be the mother of the Savior, and that creation would be worthwhile because of our Lady’s submission to God’s will.
We must to be Catholic believe that God removed from the Blessed Mother even the stain of original sin from the very moment of her conception as a singular grace based on the anticipation of our Lord’s Passion and death on the Cross. Our Lady was conceived in the normal manner by the marriage act between SS. Anne and Joachim. (There are a couple of “seers” who deny this. once again, showing the dangers of placing apparitions and seers over the teaching authority of the Church)