Ad diem illum laetissimum is an encyclical of Pope Pius X on the Immaculate Conception, dated 2 February 1904, in the first year of his Pontificate.

Mary sitteth at the right hand of her Son - a refuge so secure and a help so trusty against all dangers that we have nothing to fear or to despair of under her guidance, her patronage, her protection.

"[3] In light of his pontifical motto, “Omnia restaurare in Christo” to restore everything in Christ, Pius X in the encyclical promotes 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."

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 straightaway 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".