When did Mary agree to be Jesus' mother?

Mary was probably chosen to be the mother of Jesus long before she was even conceived. The decision was confirmed at the Annunciation when Archangel Gabriel announced to Mary that she had been chosen by God to be the human mother of His son and Mary replied, "Let it be done unto me according to thy Word." The event is commemorated each year on March 25, the feast of the Annunciation. The date of the actual event is unknown.