This Sunday's feast is curious in that it has undergone a number of name changes in recent years - yet retained its key text, viz St Luke 2:21, which describes the circumcision of Our Lord.
Traditionally, this Sunday would have been the Feast of the Circumcision of Our Lord. The feast celebrated the first time the blood of Christ was shed, and thus the beginning of the process of the redemption of man. It also serves to demonstrate that Christ was fully human, and his obedience to Biblical law.
In the 1962 Calendar (including the Benedictine Universal Calendar), all of the traditional texts for the feast are retained, but the name is dropped in favour of the Octave Day of the Nativity.
In the Novus Ordo calendar, it has become the Feast of Mary, Mother of God, apparently for reasons of ecumenism (with the Eastern Orthodox).
A rose by any other name would smell as sweet...