God gave her the gift she would need to bear, to birth and to raise the Christ.
This feast, which has been celebrated since at least the 7th century, is a celebration of the courtesy and consideration, and, yes, the mercy, of God. (It has nothing to do with how she was conceived. That came about through the union of her parents, whom the tradition names as Anna and Joachim.) The church teaches that Mary was without sin (or immaculate) since the moment of her conception. She is sinless not because, like Jesus, she is divine. She is sinless because God bestowed that grace upon her. God gave her the gift she would need to bear, to birth and to raise the Christ.
We know in our own lives, and in the lives of others, how people are given the gifts they need, often before they know of their need. They look back and only then understand how God equipped them for a task, a challenge, or a struggle.
So it was with Mary. We do not believe that her immaculate conception meant she never cried or whined as a child. She grew and learned, as all children do. It means that, as she grew, she grew more and more in the direction and likeness of God. Like a flower turning towards the sun, so Mary, in her immaculate nature, turned towards God. She said, “Yes,” and again, “Yes,” to God until she was able to say “Yes,” to the Incarnation and “Yes,” to the Passion and death of her only, beloved Son.
It is this grace, or gift of God, that the angel Gabriel understands when he addresses Mary and calls her “full of grace.” She is full of God’s grace; there is no room for sin.
Pray theses praises with St. Francis of Assisi today. He wrote this hymn in the 13th century,
Hail, O Lady, holy Queen,
Mary, holy Mother of God:
You are the Virgin made Church,
And the one chosen by the most holy Father in heaven
Whom he consecrated with his most holy beloved Son
And with the Holy Spirit the Paraclete,
In whom there was and is
All the fullness of grace and every good.
Hail, his Palace!
Hail, his Tabernacle!
Hail, his Home!
Hail, his Robe!
Hail, his servant!
Hail, his Mother!
And hail all you holy virtues
Which through the grace and light of the Holy Spirit
Are poured into the hearts of the faithful
So that from their faithless state
You may make them faithful to God.