It is celebrated on 8 December, nine months before the Nativity of Mary, which is celebrated on 8 September.
A feast called the Conception of Mary arose in the Eastern Church in the seventh century (prior to the Great Schism of 1054).
It looked to the West in the eighth century.
In the eighth century it became a feast of the Roman Catholic Church.
It is the only one of Mary's feasts that came to the Western Church not by way of Rome, but instead spread from the Byzantine area to Naples, and then to Normandy during their period of dominance over southern Italy.
From there it spread into England, France, Germany, and eventually Rome.
Prior to Pope Pius IX's definition of the Immaculate Conception as Church 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.
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.
The proper title for the feast of the Conception of the Blessed Virgin Mary in the Medieval Sarum Missal, perhaps the most famous in England, merely addresses the action of her conception.
The collect for the feast reads:
O God, mercifully hear the supplication of thy servants who are assembled together on the Conception of the Virgin Mother of God, may at her intercession be delivered by Thee from dangers which beset us.
In 1854,Pius IX made the statement Ineffabilis Deus: "The most Blessed Virgin Mary, in the first instant of her conception, by a singular grace and privilege granted by almighty God, in view of the merits of Jesus Christ, the saviour of the human race, was preserved free from all stain of original sin."
It is a public holiday in Austria, Nicaragua, Chile, Spain, Portugal, Italy, Malta, Peru, and Paraguay.
In some countries, though December 8 is not a public holiday, their respective Bishops' Conference however declared this day as a Holy Day of Obligation, as it is in the United States, the Philippines and Ireland.
December 8th is also celebrated as mother's day in Panama in honor of this holiday and is therefore a national holiday.
In the Anglican Communion, the "Conception of the Blessed Virgin Mary" may be observed as a Lesser Festival on 8 December.
Many Anglo-Catholic parishes observe the feast using the traditional Roman Catholic title, the "Immaculate Conception of the Blessed Virgin Mary."
While the Eastern Orthodox Churches have never accepted the Roman Catholic dogma of the Immaculate Conception, they do celebrate December 9 as the Feast of the Conception by St. Anne of the Most Holy Theotokos.
While the Orthodox believe that the Virgin Mary was, from her conception, filled with every grace of the Holy Spirit, in view of her calling as the Mother of God, they do not teach that she was conceived without original sin as their understanding of this doctrine differs from the Roman Catholic articulation.
The Orthodox do affirm that Mary is "all-holy" and never committed a personal sin during her lifetime.
The Orthodox feast is not a perfect nine months before the feast of the Nativity of the Theotokos September 8) as it is in the West, but a day later.