At the Annunciation, Jesus is bodily present on this earth for the first time. Mary's yes was the vehicle for that. St. Maximilian Kolbe calls her the spouse of the Holy Spirit.
There were times when I used to wonder what would have happened if Mary had hesitated, or said no. Sinless though she was, she was still 100% human. Her purity only made it possible for her to be completely open to God, but she still was not forced or programmed. I read in the Magnificat that her yes sprang from an absence of cynicism, and an acceptance of possibility in the face of what seemed impossible.
As limited as my understanding of Mary is, the Annunciation is to me the pivotal moment in history that brings the presence of Jesus alive on earth, and begins the great event of His sacrifice on our behalf. Mary had, if you will, a front row seat to WWJD for 33 years. We adore His presence at Adoration and each Sunday; she was in His presence almost continually. She was already sinless, but all of that proximity to her divine son must have had a radiating effect. As standing in the sun would have a physical effect.
St. Louis de Montfort, in his prayer to Mary, includes this passage:
The only grace I beg thee to obtain for me is that every day and every moment of my life I may say: Amen, So be it--to all that thou didst do while on earth; Amen, so be it--to all that thou art now doing in Heaven; Amen, so be it--to all that thou art doing in my soul, so that thou alone mayest fully glorify Jesus in me for time and eternity. Amen.
I say the three Amens often, after receiving our Lord in the Eucharist.
I rest in the fact that I don't have to know everything in order to trust in everything. Mary only ever leads to Jesus. Jesus only ever leads to the Father. The Father's only desire is to see us happy with Him in heaven forever.
Happy Feast day!