Sunday, March 25 is the traditional day chosen to celebrate the moment the Archangel Gabriel came down to Mary to tell her of the sacred mystery she was to participate in. A quick bit of math shows that this day is 9 months before Christmas. This year, we move the feast ahead a day, as a Sunday in Lent must be celebrated. (You may recall that in one of the last couple years the 25th was in Holy Week, and was transferred all the way past Easter)
In reality, Mary's "yes" is the reflection of Christ's own "yes" when he entered the world, as is noted in the Letter to the Hebrews in an interpretation of Psalm 39: "As is written of me in the scroll, Behold, I come to do your will, O God" (Hebrews 10:7). The Son's obedience is reflected in the Mother's and thus, by the meeting of these two "yeses," God was able to take on a human face. This is why the annunciation is also a Christological feast, because it celebrates a central mystery of Christ: his incarnation.The Pope went on to unify the "yes" of Christ and Mary with the "yes" of the saints, and especially the Martyrs. We should take this as an example for ourselves. The prayer to remember is that of our Lord "...thy Kingdom come, thy will be done...". We must remember to submit to God.
Keep this in your heart as we celebrate this great feast of Our Lady.