This feast is a logical follow-up to the Assumption and now is celebrated on the octave day of that feast. In his 1954 encyclical To the Queen of Heaven, Pius XII says that Mary deserves the title because she is Mother of God, because she is closely associated as the New Eve with Jesus’ redemptive work, because of her preeminent perfection, and because of her intercessory power.
Hail, Mary, full of grace,
the Lord is with thee.
Blessed art thou among women
and blessed is the fruit of thy womb, Jesus.
Holy Mary, Mother of God,
pray for us sinners,
now and at the hour of our death.
Amen.