The Catholic Church celebrates the Immaculate Conception of the Blessed Virgin Mary on December 8.
According to the Catechism of the Catholic Church (490-492):
“To become the mother of the Savior, Mary ‘was enriched by God with gifts appropriate to such a role.’ The angel Gabriel at the moment of the Annunciation salutes her as ‘full of grace.’ In fact, in order for Mary to be able to give the free assent of her faith to the announcement of her vocation, it was necessary that she be wholly borne by God's grace.
“Through the centuries the Church has become ever more aware that Mary, ‘full of grace’ through God, was redeemed from the moment of her conception. That is what the dogma of the Immaculate Conception confesses, as Pope Pius IX proclaimed in 1854:
The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Savior of the human race, preserved immune from all stain of original sin.
“The ‘splendor of an entirely unique holiness’ by which Mary is ‘enriched from the first instant of her conception’ comes wholly from Christ: she is "redeemed, in a more exalted fashion, by reason of the merits of her Son…’”
Here are seven quotes from the saints that magnify the importance of the Immaculate Conception of Mary and her role in God’s plan for salvation:
“This important Marian feast occurs during Advent, a season of watchful and prayerful preparation for Christmas. She who knew better than anyone how to wait attentively for the Lord guides us and shows us how to make more vital and active our journey to the Holy Night of Bethlehem. With her, we spend these weeks in prayer and, guided by her bright star, hasten to make the spiritual journey that will lead us to celebrate the mystery of the Incarnation with greater intensity.” - Saint John Paul II
"The Son of God became man for our salvation but only in Mary and through Mary." - Saint Louis Marie de Montfort
"As soon as she [Mary] had the use of reason, that is, from the first moment of her immaculate conception in the womb of Saint Ann, from that time she began with all her powers to love her God; and thus she continued to do, ever advancing more in perfection and love through her whole life. All her thoughts, her desires, her affections, were wholly given to God; not a word, not a motion, not a glance of the eye, not a breath of hers that was not for God and for his glory, never departing one step, nor separating herself for one moment from the divine love." - Saint Alphonsus Liguori
“The Immaculate Conception shines like a beacon of light for humanity in all the ages. - Saint John Paul II
"Prayer is powerful beyond limits when we turn to the Immaculata who is Queen even of God's heart." - Saint Maximilian Kolbe
"The day of the Nativity of the Mother of God is a day of universal joy, because through the Mother of God, the entire human race was renewed, and the sorrow of the first mother, Eve, was transformed into joy." - Saint John Damascene
“Today the Church is celebrating the Immaculate Conception of the Blessed Virgin Mary. If Christ is the day that never fades, Mary is its dawn, shining with beauty. Chosen in advance to be the Mother of the incarnate Word, Mary is at the same time the first-fruits of his redeeming action. The grace of Christ the Redeemer acted in her in anticipation, preserving her from original sin and from any contagion of guilt.” - Saint John Paul II
You can find a beautiful prayer for this day written by Pope Francis here.