When, in the thirteenth century, the Waldenses and Albigenses... spread false doctrines concerning the Blessed Sacrament, divine Providence ordained that, in opposition to these errors, a public profession of faith should be made by all Christendom. The will of God was made known to an obscure and pious religious named Juliana who lived near Liege. This humble and devout person had been privileged to behold in her lifetime heavenly mysteries... In a vision she saw the full moon in its splendor, one dark spot only marring the brilliance of its orb. It was revealed to her that this spot denoted the absence of a festival which should be devoted exclusively to honoring the Most Holy Sacrament. At the same time it was enjoined upon her to tell this to the bishop, and suggest that he should celebrate such a feast with the clergy of his diocese. Another religious had a similar vision about the same time, and also a recluse of the name of Eva. The bishop approved the idea of a festival with this object as tending to promote the glory of God and good of souls, and instituted it in his diocese. The new festival commended itself to all the faithful, and in 1264 Pope Urban IV, who had formerly been archdeacon of Liege, made it binding upon the whole Church...

The festival is called the feast of Corpus Christi, the body of Christ... It was for this day that St. Thomas of Aquinas composed the beautiful hymn Lauda Sion, which is recited after the epistle. At the conclusion of this sequence it is customary in some places to expose on this day the Bread of Angels in the monstrance, and give the blessing with it. After High Mass there is a solemn procession, in which the highest ecclesiastic present carries the sacred Host beneath a canopy, accompanied by the clergy.

An Explanation Of The Holy Sacraments
Rev. H. Rolfus, D.D