It is the Feast of the Immaculate Conception, which is not the same as the Virgin Birth. That would Christmas. Nor is it the same as the Annunciation, which would be 25 March when the Angel announces to the Blessed Virgin Mary that she will be with child, if she agrees.
Prior to Pope Pius IX's definition of the Immaculate Conception as a Roman Catholic dogma in 1854, most missals referred to it as the Feast of the Conception of the Blessed Virgin Mary. The festal texts of this period focused more on the action of her conception than on the theological question of her preservation from original sin. A missal published in England in 1806 indicates the same collect for the feast of the Nativity of the Blessed Virgin Mary was used for this feast as well.Regards — Cliff
The first move towards describing Mary's conception as "immaculate" came in the eleventh century. In the fifteenth century Pope Sixtus IV, while promoting the festival, explicitly tolerated those who promoted it as the Immaculate Conception and those who challenged such a description, a position later endorsed by the Council of Trent.