But that the remembrance of so great a favor might remain with us, he left to be taken by the faithful, under the appearance of bread and wine, his body for food and his blood for drink. O precious and wonderful banquet, health-giving and full of all delight! For what can be more precious than this banquet, in which not the flesh of calves and goats, as in the old law, but Christ true God, is set before us to eat? What is more wonderful than this Sacrament? For in it the substance of the bread and wine is changed into the body and blood of Christ, and therefore Christ, perfect God and man, is contained under the appearance of a little bread and wine.
He is therefore eaten by the faithful, but in no way is he mangled. Indeed when the Sacrament is divided, he remains whole under each particle. The accidents, however, remain here without any subject. And this, in order that faith may be exercised when what is visible is invisibly received, hidden under another appearance; furthermore, that the senses, which judge of the accidents according to appearances, may be preserved from error. No sacrament is more health-giving than this one, in which sins are cleansed, virtues increased, and the mind enriched with abundance of all spiritual gifts. It is offered in the Church for the living and the dead, so that what was instituted for the salvation of all, may profit all.
Finally, no one can adequately express the sweetness of this sacrament, in which spiritual sweetness is tasted in its source, and the memory is recalled of that most excellent love that Christ showed in his passion. Therefore, to impress the immensity of this love more deeply on the hearts of the faithful, at the Last Supper, after celebrating the Passover with his disciples, and about to leave this world and go to the Father, he instituted this sacrament as a lasting memorial of his passion. It fulfilled the foreshadowing of ancient rites, and was the greatest of the miracles he worked, which he left as a unique comfort to his disciples saddened by his absence.”
For the devotion of the faithful, it is good solemnly to recall the institution of such a saving and wonderful sacrament, so that we may venerate the ineffable divine presence in a visible sacrament, and praise God's power which works so many miracles in the same sacrament, and give God due thanks for such a medicinal and tasty benefit. Even though on Holy Thursday, when this sacrament was instituted, there is special mention of it in the solemn celebration of the Mass, the whole remaining part of the day's office is about the passion of Christ, which the Church is busy venerating at that time.
Therefore, to let the faithful people solemnly recall the institution of such a great sacrament with a whole Office of celebration, Pope Urban IV, out of devotion to this sacrament, decreed that the memory of this institution be celebrated by all the faithful on the first Thursday after the Octave of Pentecost. For, even though we used this sacrament for our salvation throughout the year, we can specially call to mind its institution at that time when the Holy Spirit taught the hearts of the disciples fully to understand the mysteries of this sacrament. For at that time this sacrament came into use among the faithful. For we read in the Acts of the Apostles that "they devoted themselves to the teaching of the Apostles and communing in the breaking of bread and in prayer" right after the sending of the Holy Spirit.