The Feast of the Immaculate Conception, in its oldest form, goes back to the seventh century, when churches in the East began celebrating the Feast of the Conception of Saint Anne, the mother of Mary.
In other words, this feast celebrates the conception of the Blessed Virgin Mary in the womb of Saint Anne; and nine months later, on September 8, we celebrate the Nativity of the Blessed Virgin Mary.
As originally celebrated (and as still celebrated in the Eastern Orthodox Churches), however, the Feast of the Conception of Saint Anne does not have the same understanding as the Feast of the Immaculate Conception has in the Catholic Church today.
The feast arrived in the West probably no earlier than the 11th century, and at that time, it began to be tied up with a developing theological controversy. Both the Eastern and the Western Church had maintained that Mary was free from sin throughout her life, but there were different understandings of what this meant.
