From eternity, God had Mary chosen as the Mother of God.  Mary became the Mother of God at the Annunciation.  It is her most exalted title.  This is the source of all her other privileges.  At the crucifixion, Christ gave His Mother to be the spiritual Mother to all men.  Thus, through her they could come to Him.  This feast is in place of the Maternity of Mary  formerly kept on October 13.

FEAST DAY-  January 1