The FEAST OF THE IMMACULATE CONCEPTION celebrates the solemn belief in the Immaculate Conception of the Blessed Virgin Mary. It is universally celebrated on December 8, nine months before the feast of the NATIVITY OF MARY which is celebrated on September 8. (Remember, the Catholic Church believes that life begins at Conception) It is one of the most important Marian feasts celebrated in the liturgical calendar of the Roman Catholic Church celebrated worldwide.
( Immaculate Conception means that we believe that Mary was born without Original Sin, which we believe all humans are born with, thanks to Adam and Eve and their sin. We believe that God would never place His son in the womb of a woman that had sin on her soul, so Mary, the intended Mother of God, was born without sin,)
By Pontifical designation and decree, it is the patronal feast day of Argentina, Brazil, Korea, Nicaragua, Paraguay, Philippines, Spain, the Untied States and Uruguay. By royal decree, it is also designated as the Patroness of Portugal. It is celebrated by the Roman Catholic Church as well as a few other closely related Christian churches. On this day since 1953, the Pope as Bishop of Rome visits the Column of the Immaculate Conception in Piazza di Spagna to offer expiatory prayers commemorating the solemn event.
The feast was first solemnized as a Holy Day of Obligation on 6 December 1708 under the Papal Bull COMMISSI NOBIS DIVINITUS by Pope Clement XI and is often celebrated with Holy Mass, parades, fireworks, processions, ethnic foods, and cultural festivities in honor of the Blessed Virgin Mary and is generally considered a Family day, especially in many Catholic countries. from Wikipedia
(I would add that Dec. 8 is considered a Holy Day of Obligation in the United States, which means that we Catholics are expected to attend Mass on that day. And I must report that our church was almost a full as it is on Sunday, which shows how important this Feast Day is to us.)