O Maria Sine Labe Concepta

I will greatly rejoice in the Lord, and my soul shall be joyful in my God:
for He hath clothed me with the garment of salvation,
and with the robe of justice He hath covered me,
as a bride adorned with her jewels. 
- Introit: Solemnity of the Immaculate Conception (Isaiah 61:10)

From the very beginning, and before time began, the eternal Father chose and prepared for his only-begotten Son a Mother in whom the Son of God would become incarnate and from whom, in the blessed fullness of time, he would be born into this world. Above all creatures did God so loved her that truly in her was the Father well pleased with singular delight. Therefore, far above all the angels and all the saints so wondrously did God endow her with the abundance of all heavenly gifts poured from the treasury of his divinity that this mother, ever absolutely free of all stain of sin, all fair and perfect, would possess that fullness of holy innocence and sanctity than which, under God, one cannot even imagine anything greater, and which, outside of God, no mind can succeed in comprehending fully.

The flesh of the Virgin, although derived from Adam, did not contract the stains of Adam, and that on this account the most Blessed Virgin was the tabernacle created by God himself and formed by the Holy Spirit .. They affirmed that the same Virgin is, and is deservedly, the first and especial work of God, escaping the fiery arrows the the evil one; that she is beautiful by nature and entirely free from all stain; that at her Immaculate Conception she came into the world all radiant like the dawn. For it was certainly not fitting that this vessel of election should be wounded by the common injuries, since she, differing so much from the others, had only nature in common with them, not sin. In fact, it was quite fitting that, as the Only-Begotten has a Father in heaven, whom the Seraphim extol as thrice holy, so he should have a Mother on earth who would never be without the splendor of holiness.

All our hope do we repose in the most Blessed Virgin — in the all fair and immaculate one who has crushed the poisonous head of the most cruel serpent and brought salvation to the world: in her who is the glory of the prophets and apostles, the honor of the martyrs, the crown and joy of all the saints; in her who is the safest refuge and the most trustworthy helper of all who are in danger; in her who, with her only-begotten Son, is the most powerful Mediatrix and Conciliatrix in the whole world; in her who is the most excellent glory, ornament, and impregnable stronghold of the holy Church; in her who has destroyed all heresies and snatched the faithful people and nations from all kinds of direst calamities; in her do we hope who has delivered us from so many threatening dangers. 

By the inspiration of the Holy Spirit, for the honor of the Holy and undivided Trinity, for the glory and adornment of the Virgin Mother of God, for the exaltation of the Catholic Faith, and for the furtherance of the Catholic religion, by the authority of Jesus Christ our Lord, of the Blessed Apostles Peter and Paul, and by our own: “We declare, pronounce, and define that the doctrine which holds that the most Blessed Virgin Mary, in the first instance 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, is a doctrine revealed by God and therefore to be believed firmly and constantly by all the faithful.
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- Excerpts from Pope Pius IX's Ineffabilis Deus (Encyclical on the Immaculate Conception)

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In Plausu
Hymn for Lauds: Solemnity of the Immaculate Conception

rWith joyful song we celebrate
New happiness unknown to earth
Conceived immaculate is she 
Who will be mother of her Lord.

O Mary, glory of the world, 
and daughter of eternal light, 
You Son preserved you from the first
From ev'ry taint, result of sin.

The guilty stain of Adam's fall
Afflicts each member of his race; 
The sinless Son whom you would bear
Redeemed you by his foreseen. 

The cruel serpent's crafty head
Is crushed beneath your virgin foot,
As David conquered with his sling
The giant arrogance and pride.

O dove of dazzling innocence,
The lowly handmaid of the Lord,
You bring his sign of clemency
The olive branch of peace and grace. 

To God the Father and the Son,
And to the Spirit, glory be,
Who gave you gifts beyond compare
Of holiness and purity. Amen.