Ad diem Illum Laetissimum is an encyclical of Saint Pope Pius X on the Immaculate Conception, Given at Rome in St. Peter's on the second day of February, 1904, in the first year of his Pontificate. It is issued in commemoration of the fiftieth anniversary of the dogma of the Immaculate conception. It is an important document, because it explains the Mariology of Saint Pope Pius X.
The first reason, why Pope Pius writes the encyclical, is his desire in the restoration of all things in Christ, which he had defined as his motto in his first Encyclical letter. To Pope Pius, there is no safer or more direct road than Mary. She is uniting all mankind in Christ.
The encyclical relied heavily on the views expressed in Saint Louis de Montfort's book True Devotion to Mary. In fact the language of both writings is strikingly similar, which is not surprising, since Saint Pius highly esteemed True Devotion and granted an Apostolic Blessing to all who read it.
Mary our Mother
- “Mary is the Mother of Christ and therefore she is our Mother also”. 
The encyclical states, that Christ is the Word made Flesh and the Savior of mankind. He had a physical body like any other man: But, as Savior of the human family, He had a spiritual and mystical body, the Church. The Blessed Virgin did not conceive the Eternal Son of God merely in order that He might be made man, only taking His human nature from Her. In addition, by means of the nature assumed from Her, He might be the Redeemer of men. Mary, carrying the Savior within Her, also carried all those whose life was contained in the life of the Savior. Therefore all the faithful united to Christ, are members of His body, of His flesh, and of His bones ( from the womb of Mary like a body united to its head. Though a spiritual and mystical fashion, they are all children of Mary, and She is their Mother. Mother, spiritually indeed, but truly Mother of the members of Christ. (
To restore everything in Christ through Mary
|Dogmas and Doctrines|
In light of his pontifical motto, “Omnia restaurare in Christo” to restore everything in Christ, Saint Pius X promotes in his encyclical Ad diem illum numerous worldwide Marian devotions, stating, that “no honouring is more pleasing to Mary, none She likes better, than the one in which we truly recognize and love Jesus. This is according to Pius X the aim of Marian festivities. If the faithful engage in festivities in their churches, if parishes organize feast, this is fine according to the pontiff. This may indeed promote piety. But Marian veneration has to reach the innermost nucleus, and not be limited in outward festivities. Otherwise it would be a façade of real religiosity.
- Unless heart and will be added, they will all be empty forms, mere appearances of piety. At such a spectacle, the Virgin, borrowing the words of Jesus Christ, would address us with the just reproach: "This people honoureth me with their lips, but their heart is far from me" (Matth. xv., 8).
Whoever wishes, that his devotion should be worthy of Her, should go further and strive might and main to imitate Her example. The Pontiff declares, that those only attain everlasting happiness who have by such following reproduced in themselves the patience and sanctity of Jesus Christ.
The Immaculate Conception
Many modern men deny that mankind has fallen by sin, believing that the concept original sin and all the evils, that were allegedly its consequence, are pure imaginations. With this rejection, there is logically “no place left for Christ, for the Church, for grace or for anything that is above and beyond nature; in one word the whole edifice of faith is shaken from top to bottom. "But let people believe and confess that the Virgin Mary has been from the first moment of her conception preserved from all stain; and it is straightway necessary that they should admit both original sin and the rehabilitation of the human race by Jesus Christ, the Gospel, and the Church and the law of suffering”  The dogma of the Immaculate Conception is the answer, by its obligation “of recognizing in the Church a power before which (mankind ) not only has the will to bow, but the intelligence to subject itself”  The Virgin, so Pope Pius X, was kept the more free from all stain of original sin because she was to be the Mother of Christ; and She was the Mother of Christ, so that the hope of everlasting happiness might be born again in our souls 
- Ad Diem Illum at the Vatican website