Source the old Catholic Enyclopedia
This feast is celebrated in the Latin Church on the Thursday after Trinity Sunday to solemnly commemorate the institution of the Holy Eucharist.
Of Maundy Thursday, which commemorates this great event, mention is made as Natalis Calicis (Birth of the Chalice) in the Calendar of Polemius (448) for the 24th of March, the 25th of March being in some places considered as the day of the death of Christ. This day, however, was in
Holy Week, a season of sadness, during which the minds of the faithful are expected to be occupied with thoughts of the Lord’s Passion. Moreover, so many other functions took place on this day that the principal event was almost lost sight of. This is mentioned as the chief reason for the introduction of the new feast, in the Bull “Transiturus.”
The instrument in the hand of Divine Providence was St. Juliana of Mont Cornillon, in Belgium. She was born in 1193 at Retines near Liège. Orphaned at an early age, she was educated by the Augustinian nuns of Mont Cornillon. Here she in time made her religious profession and later became superioress. Intrigues of various kinds several times drove her from her convent. She died 5 April, 1258, at the House of the Cistercian nuns at Fosses, and was buried at Villiers.
Juliana, from her early youth, had a great veneration for the Blessed Sacrament, and always longed for a special feast in its honour. This desire is said to have been increased by a vision of the Church under the appearance of the full moon having one dark spot, which signified the absence of such a solemnity. She made known her ideas to Robert de Thorete, then Bishop of Liège, to the learned Dominican Hugh, later cardinal legate in the Netherlands, and to Jacques Pantaléon, at that time Archdeacon of Liège, afterwards Bishop of Verdun, Patriarch of Jerusalem, and finally Pope Urban IV. Bishop Robert was favourably impressed, and, since bishops as yet had the right of ordering feasts for their dioceses, he called a synod in 1246 and ordered the celebration to be held in the following year, also, that a monk named John should write the Office for the occasion. The decree is preserved in Binterim (Denkwürdigkeiten, V, 1, 276), together with parts of the Office.
Bishop Robert did not live to see the execution of his order, for he died 16 October, 1246; but the feast was celebrated for the first time by the canons of St. Martin at Liège. Jacques Pantaléon became pope 29 August, 1261. The recluse Eve, with whom Juliana had spent some time, and who was also a fervent adorer of the Holy Eucharist, now urged Henry of Guelders, Bishop of Liège, to request the pope to extend the celebration to the entire world. Urban IV, always an admirer of the feast, published the Bull “Transiturus” (8 September, 1264), in which, after having extolled the love of Our Saviour as expressed in the Holy Eucharist, he ordered the annual celebration of Corpus Christi in the Thursday next after Trinity Sunday, at the same time granting many indulgences to the faithful for the attendance at Mass and at the Office. This Office, composed at the request of the pope by the Angelic Doctor St. Thomas Aquinas, is one of the most beautiful in the Roman Breviary and has been admired even by Protestants.
The death of Pope Urban IV (2 October, 1264), shortly after the publication of the decree, somewhat impeded the spread of the festival. Clement V again took the matter in hand and, at the General Council of Vienne (1311), once more ordered the adoption of the feast. He published a new decree which embodied that of Urban IV. John XXII, successor of Clement V, urged its observance.
Neither decree speaks of the theophoric procession as a feature of the celebration. This procession, already held in some places, was endowed with indulgences by Popes Martin V and Eugene IV.
The feast had been accepted in 1306 at Cologne; Worms adopted it in 1315; Strasburg in 1316. In England it was introduced from Belgium between 1320 and 1325. In the United States and some other countries the solemnity is held on the Sunday after Trinity.
It is also noted that in Canada and the US we now celebrate the Feast of Corpus Christi on the 2nd Sunday after Pentecost instead of the Thursday after Trinity Sunday.
“In 1970 Paul VI merged the feast of the Most Holy Blood of Our Lord, which was celebrated on July 1st as a fixed feast, with the feast of Corpus Christi. The feast of the Sacred Heart of Jesus is also a moveable feast celebrated Friday after the octave of Corpus Christi. In 1856 Pius IX declared the Sacred Heart a feast day for the entire Church; notwithstanding, the American Bishops did not include it as a day of obligation.”