The Mother of the Redeemer has a precise place in the plan of salvation, for "when the time had fully come, God sent forth his Son, born of woman, born under the law, to redeem those who were under the law, so that we might receive adoption as sons. And because you are sons, God has sent the Spirit of his Son into our hearts, crying, 'Abba! Father!'" (Gal. 4:4-6)
... For they are words which celebrate together the love of the Father, the mission of the Son, the gift of the Spirit, the role of the woman from whom the Redeemer was born, and our own divine filiation, in the mystery of the "fullness of time."
This "fullness" indicates the moment fixed from all eternity when the Father sent his Son "that whoever believes in him should not perish but have eternal life" (Jn. 3:16).
It denotes the blessed moment when the Word that "was with God... became flesh and dwelt among us" (Jn. 1:1, 14), and made himself our brother.
It marks the moment when the Holy Spirit, who had already infused the fullness of grace into Mary of Nazareth, formed in her virginal womb the human nature of Christ.
This "fullness" marks the moment when, with the entrance of the eternal into time, time itself is redeemed, and being filled with the mystery of Christ becomes definitively "salvation time."
Finally, this "fullness" designates the hidden beginning of the Church's journey. In the liturgy the Church salutes Mary of Nazareth as the Church's own beginning, for in the event of the Immaculate Conception the Church sees projected, and anticipated in her most noble member, the saving grace of Easter. And above all, in the Incarnation she encounters Christ and Mary indissolubly joined: he who is the Church's Lord and Head and she who, uttering the first fiat of the New Covenant, prefigures the Church's condition as spouse and mother.
Saint John Paul II, Redemptoris mater §1.