There are some erroneous concepts that need to be cleared up about this day, the Feast of the Immaculate Conception:
1) It's not Jesus's conception. Even Catholics get this wrong. Jesus's miraculous conception is honored on March 25 (the Feast of the Annunciation)---reasonably, nine months before his birthday is celebrated. It gets confusing, because with Christmas coming up, we're thinking about Jesus's babyhood. The Immaculate Conception is the day Mary was conceived, as she was born without Original Sin, unlike the rest of us slobs. But why is Mary's conception celebrated so close to Christmas---especially since we don't really know what dates any of these took place?
2) I'm not sure why December 8 was chosen, actually; a reading of the history of the feast shows that it arose slowly in the monasteries, and other dates were used at times. All I can say is that it should be celebrated and 12/8 had a 1 in 365.25 chance of being precisely right -- if you presume divine guidance along the way, the chances improve immeasurably.
Interestingly, the Annunciation is not a US day of obligation, meaning Catholics are not obliged to attend Mass that day, but the Feast of the Immaculate Conception is. Why?
3) Everyone knows that Patrick is the patron saint of Ireland, that George is the patron saint of England, that Andrew is the patron saint of Scotland, and that Archangel Michael is the patron saint of Papua New Guinea. (What, you didn't know that?) Well, the Virgin Mary, as Our Lady of the Immaculate Conception, is the patron saint of the United States of America. (A lot of Catholics don't know that, either.) (Brazil, too.) So it's not just a day of commemorating the Queen of the Saints; it's a national holiday in this country as well. Which is why, unlike other days of obligation like the Assumption of Mary into heaven (August 15) and All Saints Day (November 1), which may be abrogated in the U.S. if they fall on a Saturday or Monday -- in other words, the feast is moved to the neighboring Sunday -- the Feast of the Immaculate Conception always requires church attendance in America regardless of the day of the week upon which December 8 falls.
In the United States, and indeed throughout the Western Hemisphere, devotion to Mary is extremely strong. Her appearance at Guadelupe occured on December 9, 1531, when the church had barely arrived here. In 1859 she appeared three times in Wisconsin---really.
The church does not accept these things easily, by the way, so don't go running to the local parish because you see Mary in a potato chip. It is far easier for you or me to become a publicly proclaimed saint than to have a claim of an apparition from Mary accepted by Rome. It's true! We don't have to walk on water or chase all the snakes out of Cleveland; we just have to get murdered while preaching the Gospel. There's a lot of that about these days. Fans of Dan Brown's novels may think that people are getting killed because of the Catholic church's secrets, but in fact people are getting killed because of Christianity's very public knowledge.
On this feast day, I, as a Catholic, suggest you go visit your parish if you're Catholic, or even if you're not---what could it hurt? You might be inspired by it. You might even enjoy it.