The origin of the Feast of Corpus Christi began with a Eucharistic miracle, which is a moment of divine intervention aimed at confirming the faith in the Real Presence of the Body and Blood of Jesus in the Eucharist. Here is a description of the event in 1263:
In 1263 a German priest, Peter of Prague, stopped at Bolsena [Italy] while on a pilgrimage to Rome. He... [was] a pious priest, but one who found it difficult to believe that Christ was actually present in the consecrated Host. While celebrating Holy Mass above the tomb of St. Christina (located in the church named for this martyr), he had barely spoken the words of Consecration when blood started to seep from the consecrated Host and trickle over his hands onto the altar and the corporal. The priest was immediately confused. At first, he attempted to hide the blood, but then he interrupted the Mass and asked to be taken to the neighboring city of Orvieto, where Pope Urban IV resided. The Pope listened to the priest's account and absolved him. He then sent emissaries for an immediate investigation. When all the facts were ascertained, he ordered the Bishop of the diocese to bring the Host and the linen cloth bearing the stains of blood to Orvieto. With archbishops, cardinals and other Church dignitaries in attendance, the Pope met the procession and, amid great pomp, had the relics placed in the cathedral. The linen corporal bearing the spots of blood is still reverently enshrined and exhibited in the Cathedral of Orvieto. It is said that Pope Urban IV was prompted by this miracle to commission St. Thomas Aquinas to compose the Proper for a Mass and an Office honoring the Holy Eucharist as the Body of Christ. One year after the miracle, in August of 1264, Pope Urban IV introduced the saint's composition, and by means of a papal bull instituted the feast of Corpus Christi.
This was when St. Thomas Aquinas wrote the hymn containing the verses of Tantum Ergo. The papal bull, Transiturus, ordered Corpus Christi be celebrated annually on the Thursday after Trinity Sunday throughout the Latin Church. In modern times, this feast is commonly transferred to the following Sunday to accommodate the faithful.
In August of 1964, on the 700th anniversary of the institution of the feast of Corpus Christi, Pope Saint Paul VI celebrated Holy Mass at the altar where the holy corporal is kept in its golden shrine in the Cathedral of Orvieto.
Video Link: Eucharistic Miracle: Chapel of the Miracle, Orvieto, Italy
https://youtu.be/OEq2JAszRWA?si=-gIHpbwhATTUAUVw
https://youtu.be/OEq2JAszRWA?si=I0fSvCDzl3JYRluPhttps://youtu.be/OEq2JAszRWA?si=I0fSvCDzl3JYRluP
