Holy Scripture reveals to us that Saturday is the day when creation was completed and so is celebrated as the day of the fulfillment of the plan of salvation, which found its realization through Mary. Sunday is the Lord’s Day, so it is appropriate to observe the preceding day as Mary’s day.
In addition, as the book of Genesis describes, God rested on the seventh day, Saturday. The seventh day, Saturday, is the Jewish Sabbath. But we as Christians rest on Sunday, because we celebrate the Resurrection as our Sabbath Day. In parallel, Jesus rested in the womb and then in the loving arms of Mary from birth until she held His lifeless body at the foot of the Cross; thus God Himself rested in Mary before His birth and before His resurrection.
And there is a further tradition: it is a remembrance of the maternal example and discipleship of the Blessed Virgin Mary who, strengthened by faith and hope, on that great Saturday on which Our Lord lay in the tomb, held vigil in expectation of the Lord’s resurrection. And so it is a prelude and introduction to the celebration of Sunday, the weekly memorial of the Resurrection of Christ. Indeed, it is a sign that the Virgin Mary, Mother of God, is continuously present and active in the life of the Church.