There's Something About Mary - A Primer on the Immaculate Conception
Who: The Blessed Mother, also known as Mary, Our Lady, Theotokos, the one whom "all generations will called 'blessed'", and the one who is "full of grace"
What: It was declared, pronounced and defined “that the doctrine which asserts that the Blessed Virgin Mary, from the first moment of her conception, by a singular grace and privilege of almighty God, and in view of the merits of Jesus Christ, Saviour of the human race, was preserved free from every stain of original sin is a doctrine revealed by God and, for this reason, must be firmly and constantly believed by all the faithful”.
Where: In the Papal encyclical Ineffabilis Deus, by Pope Pius IX
When: December 8, 1854 ; also venerated by St. Ephrem the Syrian (d. AD 373), St. Ambrose (d. AD 397), St. John of Damascus (d. AD 755), Blessed John Duns Scotus (d. 1308), The Council of Basle in 1439), Pope Sixtus IV in 1476, Pope Clement XI in 1708, and Pius IX in 1847.
Why: The immunity from original sin was given to Mary by a singular exemption from a universal law through the same merits of Christ, by which other men are cleansed from sin by baptism. Mary needed the redeeming Saviour to obtain this exemption, and to be delivered from the universal necessity and debt (debitum) of being subject to original sin. The person of Mary, in consequence of her origin from Adam, should have been subject to sin, but, being the new Eve who was to be the mother of the new Adam, she was, by the eternal counsel of God and by the merits of Christ, withdrawn from the general law of original sin. Her redemption was the very masterpiece of Christ's redeeming wisdom. He is a greater redeemer who pays the debt that it may not be incurred than he who pays after it has fallen on the debtor. Such is the meaning of the term "Immaculate Conception." (newadvent.org)
How: It is universally celebrated in the Church on December 8, nine months prior to the Feast of the Nativity of Mary on September 8. The Solemnity of the Immaculate Conception of the Blessed Virgin Mary is a Holy Day of Obligation in the United States, Ireland, and the Philipines. It is the Patronal Feast Day of United States of America, Spain, Korea, Portugal, Nicaragua, Brazil, and the Philippines.
Readings at Mass: Genesis 3:9-15, 20; Psalm 98:1, 2-3ab, 3cd-4; Ephesians 1:3-6, 11-12; Luke 1:26-38
Aspiration to Mary: O Mary, conceived without sin, pray for us who have recourse to thee.
The Catechism of the Catholic Church: §§ 490-493
To the Virgin Immaculate
O Virgin Immaculate, Mother of God and my Mother, from thy sublime height turn upon me thine eyes of pity. Filled with confidence in thy goodness and knowing full well thy power, I beseech thee to extend to me thine assistance in the journey of life, which is so full of danger for my soul. And in order that I may never be the slave of the devil through sin, but may ever live with my heart humble and pure, I entrust myself wholly to thee. I consecrate my heart to thee for ever, my only desire being to love thy divine Son Jesus. Mary, none of thy devout servants has ever perished; may I too be saved. Amen.