Mary is called the Mother of God because her Son, Jesus Christ the Messiah is God the Son. Since Jesus is God, then the one who gave Him flesh is called Mother of God.
What an awesome thought. A human female is the Mother of the Word Incarnate. She is the Mother of God! Since the early days of the Catholic Church, devotion to Mary has existed. The "Sub Tuum Praesidium" is one of the earliest prayers to Mary.
This is a day of Holy Obligation in which all Catholics must treat it as a Sunday and attend Mass. Let us take this day to recollect on Mary and her role in Salvation. Mary was always open to life. She said Yes to God's will that she be the vessel in which God the Son would take on a human form.
She is a model for all women, especially those who push the disturbing idea that pregnancy and bearing children violates a woman's bodily autonomy or dignity. This tokophobia must be replaced with the Yes to life from the Theotokos.
May Our Lady pray for us and show us the way to Christ her Son. Mary Mother of God, Pray for us!
We take refuge under thy protection,holy Mother of God:
Do not despise our prayers
in time of necessity:
but always free us
from all dangers,
O blessed and glorious Virgin.