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.
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.