The doctrine of the Immaculate Conception maintains that the Virgin Mary was conceived free from sin, therefore ready to be the pure vessel for Christ’s birth. The Immaculate Conception enjoyed intense devotion in Spain during the 1600s, although it was only accepted as official doctrine in the 1860s. The abstract subject required artists to develop appropriate imagery. The crescent moon, for example, comes from the New Testament vision of Saint John the Evangelist (Revelation 12:1) of “a woman clothed with the sun, with the moon under her feet.
— Internet Archive

Normally, the Immaculate Conception is celebrated on December 8th.