The month of December is traditionally dedicated to the Immaculate Conception. The Blessed Virgin Mary, according to the teaching of the Catholic Church, in the first moment of her conception, by a singular privilege of Almighty God, and by virtue of the foreseen merits of Jesus Christ, our Saviour and hers, was preserved from all stain of original sin. 

The Catholic Church teaches that Mary was conceived by normal biological means in the womb of her mother, Saint Anne, but God acted upon her soul, keeping it "immaculate". This age-old belief of the Church was defined by Pope Pius IX in 1854 as an article of revealed truth.

Although the belief that Mary was sinless, or conceived with an immaculate soul, has been widely held since Late Antiquity, the doctrine was not dogmatically defined until 1854, by Pope Pius IX in his papal bull Ineffabilis Deus. The Catholic Church celebrates the Feast of the Immaculate Conception on December 8; in many Catholic countries, it is a holy day of obligation or patronal feast. 

Immaculate Conception

When discussing the Immaculate Conception, an implicit reference may be found in the angelic greeting to Mary. The Archangel Gabriel said, "Hail, full of grace, the Lord is with thee." (Luke 1:28). The phrase "full of grace" is a translation of the Greek word kecharitomene. It therefore expresses a characteristic quality of Mary. 

The traditional translation, "full of grace," is better than the one found in many recent versions of the New Testament, which give something along the lines of "highly favored daughter." Mary was indeed a highly favored daughter of God, but the Greek implies more than that (and it never mentions the word for "daughter"). The grace given to Mary is at once permanent and of a unique kind. Kecharitomene is a perfect passive participle of charitoo, meaning "to fill or endow with grace." Since this term is in the perfect tense, it indicates that Mary was graced in the past but with continuing effects in the present. So, the grace Mary enjoyed was not a result of the angel’s visit. In fact, Catholics hold, it extended over the whole of her life, from conception onward. She was in a state of sanctifying grace from the first moment of her existence. 

The Feast of Immaculate Conception

By 750, the feast of her conception was widely celebrated in the Byzantine East, under the name of the Conception (active) of Saint Anne. In the West it was known as the feast of the Conception (passive) of Mary, and was associated particularly with the Normans, whether these introduced it directly from the East or took it from English usage.

The spread of the feast, by now with the adjective "Immaculate" attached to its title, met opposition on the part of some, on the grounds that sanctification was possible only after conception. Critics included Saints Bernard of Clairvaux, Albertus Magnus and Thomas Aquinas. Other theologians defended the expression "Immaculate Conception", pointing out that sanctification could be conferred at the first moment of conception in view of the foreseen merits of Christ, a view held especially by Franciscans.

William of Ware and Blessed John Duns Scotus pointed out that Mary’s Immaculate Conception enhances Jesus’ redemptive work. One of the chief proponents of the doctrine was the Hungarian Franciscan Pelbartus Ladislaus of Temesvár. 

On 28 February 1476, Pope Sixtus IV, authorized those dioceses that wished to introduce the feast to do so, and introduced it to his own diocese of Rome in 1477, with a specially composed Mass and Office of the feast.With his bull Cum praeexcelsa of 28 February 1477, in which he referred to the feast as that of the Conception of Mary, without using the word "Immaculate", he granted indulgences to those who would participate in the specially composed Mass or Office on the feast itself or during its octave, and he used the word "immaculate" of Mary, but applied instead the adjective "miraculous" to her conception. On 4 September 1483, referring to the feast as that of "the Conception of Immaculate Mary ever Virgin", he condemned both those who called it mortally sinful and heretical to hold that the "glorious and immaculate mother of God was conceived without the stain of original sin" and those who called it mortally sinful and heretical to hold that "the glorious Virgin Mary was conceived with original sin", since, he said, "up to this time there has been no decision made by the Roman Church and the Apostolic See." This decree was reaffirmed by the Council of Trent.

Pope Pius V, while including the feast in the Tridentine Calendar, removed the adjective "Immaculate" and suppressed the existing special Mass for the feast, directing that the Mass for the Nativity of Mary (with the word "Nativity" replaced by "Conception") be used instead. Part of that earlier Mass was revived in the Mass that Pope Pius IX ordered to be used on the feast and that is still in use.

On 6 December 1708, Pope Clement XI made the feast of the Conception of Mary, at that time still with the Nativity of Mary formula for the Mass, a Holy Day of Obligation. Until Pope Pius X reduced in 1911 the number of Holy Days of Obligation to 8, there were in the course of the year 36 such days, apart from Sundays.[36] Writers such as Sarah Jane Boss interpret the existence of the feast as a strong indication of the Church's traditional belief in the Immaculate Conception.

 (Source from: https://en.wikipedia.org/wiki/Immaculate_Conception)

Novena of Immaculate Conception

This novena starts on November 29, which begins 9 days before the Feast of the Immaculate Conception.

O most pure Virgin Mary conceived without sin, from the very first instant, you were entirely immaculate. O glorious Mary full of grace, you are the mother of my God – the Queen of Angels and of men. I humbly venerate you as the chosen mother of my Savior, Jesus Christ.

The Prince of Peace and the Lord of Lords chose you for the singular grace and honor of being His beloved mother. By the power of His Cross, He preserved you from all sin. Therefore, by His power and love, I have hope and bold confidence in your prayers for my holiness and salvation.

I pray that your prayers will bring me to imitate your holiness and submission to Jesus and the Divine Will.

Hail Mary, full of Grace, the Lord is with you. Blessed are you among women and blessed is the fruit of your womb, Jesus. Holy Mary, Mother of God, pray for us sinners now and at the hour of our death.

Now, Queen of Heaven, I beg you to beg my Savior to grant me these requests

(Mention your intentions)

My holy Mother, I know that you were obedient to the will of God. In making this petition, I know that God’s will is more perfect than mine. So, grant that I may receive God’s grace with humility like you.

As my final request, I ask that you pray for me to increase in faith in our risen Lord; I ask that you pray for me to increase in hope in our risen Lord; I ask that you pray for me to increase in love for the risen Jesus!

Hail Mary, full of Grace, the Lord is with you. Blessed are you among women and blessed is the fruit of your womb, Jesus. Holy Mary, Mother of God, pray for us sinners now and at the hour of our death. Amen.