As far back as the 5th century, records can be found in the Eastern church of a feast day falling on December 9th in Syria known as the Feast of the Conception of the Most Holy and All Pure Mother of God. Records also indicate that this feast was very wide spread in the East in the 7th century. Sometime during the 8th century, the feast was translated into the Western Church with a set feast day of December 8th, spreading first from Southern Italy to Normandy and then to the rest of Europe and Rome. In his Papal Bull Commissi Nobis Divinitus in 1708, Pope Clement X changed the feast to a Holy Day of Obligation and had the bull notarized and copied for dissemination.
In 1854, Pope Pius IX issues the Apostolic constitution 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 savior of the human race, was preserved free from all stain of original sin."
This Apostolic constitution made no change of dogma but instead formally defined it. Pope Pius IX continued:
And hence they [the Church Fathers] affirmed that the Blessed Virgin was, through grace, entirely free from every stain of sin, and from all corruption of body, soul and mind; that she was always united with God and joined to him by an eternal covenant; that she was never in darkness but always in light; and that, therefore, she was entirely a fit habitation for Christ, not because of the state of her body, but because of her original grace. . . .
The Catechism explains it as this:
490 To become the mother of the Saviour, Mary “was enriched by God with gifts appropriate to such a role.” The angel Gabriel at the moment of the annunciation salutes her as “full of grace”. In fact, in order for Mary to be able to give the free assent of her faith to the announcement of her vocation, it was necessary that she be wholly borne by God's grace.
491 Through the centuries the Church has become ever more aware that Mary, “full of grace” through God, was redeemed from the moment of her conception. That is what the dogma of the Immaculate Conception confesses, as Pope Pius IX proclaimed in 1854:
The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Saviour of the human race, preserved immune from all stain of original sin.
492 The “splendour of an entirely unique holiness” by which Mary is “enriched from the first instant of her conception” comes wholly from Christ: she is “redeemed, in a more exalted fashion, by reason of the merits of her Son”. The Father blessed Mary more than any other created person “in Christ with every spiritual blessing in the heavenly places” and chose her “in Christ before the foundation of the world, to be holy and blameless before him in love”.
493 The Fathers of the Eastern tradition call the Mother of God “the All-Holy” (Panagia), and celebrate her as “free from any stain of sin, as though fashioned by the Holy Spirit and formed as a new creature”. By the grace of God Mary remained free of every personal sin her whole life long. “Let it be done to me according to your word. . .”
508 From among the descendants of Eve, God chose the Virgin Mary to be the mother of his Son. “Full of grace”, Mary is “the most excellent fruit of redemption” (SC 103): from the first instant of her conception, she was totally preserved from the stain of original sin and she remained pure from all personal sin throughout her life.
December 8th is considered a Solemnity and a Holy Day of obligation. Should it fall on a Saturday, the precept of attending Mass is still observed in the United States. If the day falls on a Sunday, it is transferred to the following Monday. This is different in the 1960 Code of Rubrics however, which states in accordance with Summorum Pontificum that the feast is given preference even over Advent Sunday.
In many countries the day is considered an official public holiday:
Italy (National holiday since 1953 ; the Pope visits the Column of the Immaculate Conception in Piazza di Spagna)
Monaco (Celebrated with food festivities and honoring of mothers and grandmothers)
Philippines (Signed into the constitution as a public holiday on March 29th, 2017 as Mary is the Patroness of the country)
Spain (Was designated by Pope Clement XIII in 1760)