(For religion and psychology nerds only)
If you want to wander into the labyrinthine ways of Catholic dogma, today is your day. December 8th marks the solemnity (first class festival) of the Immaculate Conception. People commonly confuse this idea with the Virgin Birth of Jesus or his own conception by the Holy Spirit and the Virgin, celebrated on March 25th. But this day and dogma, to be concrete, is about the moment when the Virgin Mary's parents --Joachim and Anne, below-- conceived her. Among other things, it is about the first act of sexual intercourse in history (below) which did not produce a child stained and limited by Original Sin.
Joachim & Anna "embracing" at the Golden Gate
according to the Protoevangelium of James.
Following me so far?
This belief has a long and complex history, as you might imagine. St Thomas Aquinas, among others, did not favor it. Part of the sibling rivalry between Dominicans and Franciscans. The Franciscans eventually won the argument, as noted by Gerard Manley Hopkins in his praise of Friar Minor Duns Scotus, "who fired France for Mary without spot."
Thomas was afraid that understanding Mary as free from sin long before the Passion of Christ occurred would reduce the necessity and universality of his atonement. And the Gospels give some grounds for wondering if she was really on board with Jesus' mission (Mk 3.32ff and parallels). By the time that Rome declared it a dogma of the faith in 1854, the argument was long over. The Church eventually decided that her exceptional status was proleptic, that is, although the effect happened in time before the cause, it was nonetheless an effect of that cause. See what I mean by labyrinthine?
The Collect of the day:
Deus, qui per immaculatam Virginis Conceptionem
dignum Filio tuo habitaculum praeparasti,
quaesumus, ut, qui ex morte eiusdem Filii tui praevisa,
eam ab omni labe praeservasti,
nos quoque mundos, eius intercessione,
ad te pervenire concedas.
O God, who by the Immaculate Conception of the Blessed Virgin
prepared a worthy dwelling for your Son,
grant, we pray,
that, as you preserved her from every stain
by virtue of the Death of your Son, which you foresaw,
so, through her intercession,
we, too, may be cleansed and admitted to your presence.
Goya's Virgin of the Immaculate Conception
As for the date, which conveniently falls in Advent, it was arrived at by counting back nine months from the much earlier feast (6th/7th centuries) of the Virgin's own birth, September 8th in both Western and Eastern churches. Which may have its origins in the Byzantine New Year or the rededication of a Jerusalem church in honor of St. Anne.
Why some people think I am a pointy-headed intellectual, I'll never know.
*Unless I am mistaken --hardly possible here on Ex Cathedra :) -- while the incest taboo twixt mother and son is universal among humans, incest is not at all uncommon in the mythic worlds of the gods. Kinship libido drives both the mythic aspects of Christianity as a whole (Father and Son/Hero) and Catholicism in particular (Son/Lover and Mother).
Although Muhammad's Quran makes a mistake in describing the Trinity as Father, Mother and Son, since Muhammad's knowledge of Christianity came from the 7th century variety as popularly preached and practiced, it is an understandable error. One of the reasons the Third Person of the Trinity, the Holy Spirit, has long been overshadowed by the Virgin Mother, ironically**, in Catholicism, it is because in functional and devotional terms, she largely replaced Him. An actual human woman is, I suppose, more attractive than a metaphorical dove, fire and wind. Liberation theologian Leondardo Boff even suggested that she was the the Holy Spirit's incarnation.