Observed in Countries with Roman Catholic and Episcopal populations, as well as members of the Church of England
Observed on Sixty days after Easter, on the Thursday after Trinity Sunday, and in some countries three days later, on Sunday
Observed by Christians, primarily Roman Catholic, Episcopal, and Church of England

During the Last Supper, Jesus offered bread and a cup of wine to his disciples, proclaiming these to be a sacred transformation of with his body and his blood. Jesus’ act is the basis of the festival of Corpus Christi. Corpus Christi means “the body of Christ” in Latin. Corpus Christi is a religious feast, observed in May or June on the eighth Thursday (or 60 days) after Easter. It commemorates the institution of the Holy Eucharist (the practice of consuming of bread and wine representing the body and blood of Jesus by the congregation during the rite of Communion).
Roman Catholics, some sections of the Church of England, and the Episcopal Church celebrate this festival. Its date is not fixed to a particular day of the year, but moves according to the date of Easter, which also varies from year to year.
In the early days of the Christian Church, Corpus Christi was celebrated during Holy Week. This was a somber time for Christians who meditated on the Passion and suffering of Jesus. Since many other functions were also organized during this festival, the significance of the actual event tended to be lost.
To safeguard the sanctimony of the main festival Pope Urban IV (r. 1261–64 C.E.) decided to introduce a new feast in 1264. He decreed that the festival be celebrated on the Thursday after Trinity Sunday (the first Sunday after Pentecost, which marks the descent of the Holy Spirit to Earth). St.
Thomas Aquinas (1225/27–74) was asked to write the office (the prescribed form of worship) for the festival. The feast has been celebrated in many parts of the world for the 750 years since.
After it was introduced in Belgium, the feast came to be accepted at Cologne in Germany in 1306 and in England between 1320 and 1325. In 1970 the day was changed to the Sunday after Trinity Sunday in the United States and in many other parts of the world.
Corpus Christi is not among the great feasts of Eastern Orthodoxy, but in the Greek Church, the Syrians, Armenians, Coptics, Melchites, and the Ruthenians of Galicia, Calabria, and Sicily observe the feast of Corpus Christi.

Origins and History
The origins of the festival can be traced to St. Juliana, an Augustinian nun. She was born in 1193 at Retines near Liège in Belgium. She had been orphaned early in life and was cared for and educated by the Augustinian nuns. Later she became the superior (head) of the convent. From an early age she displayed a deep devotion to the Eucharist, and this feeling was strengthened when the Lord appeared to her at the age of 16 and told her that there should be a feast to celebrate the Eucharist or Blessed Sacrament. The reasons for the feast given by the Lord were threefold: He wanted to strengthen Catholic faith in the Eucharist. He wanted people to derive strength from it and, thus, lead virtuous lives; and honoring the Eucharist would make up for any sacrilegious acts committed against the Eucharist. Another story that connects her decision to request a feast celebrating the Eucharist describes her vision as one of the Catholic Church, under a full Moon, with a single dark spot. This dark spot represented the lack of such an observance.
Twenty years after receiving this vision, Juliana became the superior of her order, and she decided that this was the time to create the feast. She spoke to the bishop of Liège Robert de Thorte, to the learned Dominican Hugh, later cardinal legate in the Netherlands, to Jacques Pantaleon, archdeacon of Liège, who later became bishop of Verdun, to the patriarch of Jerusalem, and finally to Pope Urban IV. The bishops of those times had the power to order feasts in their dioceses. In 1246 Bishop Robert ordered the feast to be observed, starting the next year, in his diocese.
When Juliana passed away in 1258, the feast had not yet spread to all parts of the world. After she was canonized as a saint, there arose some controversy over celebrating it. It was felt by certain sections of the Catholic Church that there was no need to have a separate feast to mark the Eucharist and that it was being given sufficient importance in daily church rituals. This controversy was, however, resolved by a miracle that took place in the town of Bolsena in Italy in 1263.
A German priest, Father Peter of Prague, was on his way to Rome, when he stopped in the town of Bolsena in Orvieto, Italy, to hold a Mass. He did not fully believe that Jesus was present in the Eucharist, but, as he was speaking the words of the Consecration, blood began to drip from the blessed bread onto the altar cloth below. He ceased to doubt that the bread and wine offered during the Mass were actually the body and blood of Jesus.
Amazed by this incident he went to see Pope Urban IV and described what had happened to him. After conducting his own investigation, the pope declared it a miracle and decreed the Feast of Corpus Christi to be celebrated on September 8, 1264.
In 1264 Pope Urban IV died, and this prevented the feast from spreading farther. However, in 1311 at the Council of Vienne, Pope Clement V (r. 1305–14) declared that the feast be adopted throughout the world and published a decree to this effect. Its spread was further encouraged by his successor Pope John XXII (r. 1316–34.) The procession that is a part of the feast today was not referred to in either of their decrees, though it was being done in some places. This was made a regular part of the celebration by Popes Martin V (r. 1417–31) and Eugene IV (r. 1431–47.)