The institution of the feast of Corpus Christi happened after a miraculous miracle at the time of St. Thomas Aquinas.
The Church offers us different feasts to celebrate the Eucharist. On Maundy Thursday we celebrate the institution of the Eucharist when Our Lord celebrated the first Mass and the institution of the priesthood. However the main focus of the Holy Week being the Redemption, it was fitting to have a feast dedicated only to the mystery of love which could be celebrated with great joy.
Pope Urban IV instituted the solemnity of Corpus Christi on the Thursday after Pentecost, by the papal bull Transiturus de hoc mundo (Aug. 11, 1264).
The pontiff made a point of setting an example by celebrating the first solemnity in Orvieto, the town where he was then residing. Indeed, he ordered that the famous corporal with the traces of the Eucharistic miracle that had occurred in Bolsena the previous year, 1263, be kept in Orvieto Cathedral — where it still is today.
While a priest was consecrating the bread and the wine he was overcome by strong doubts about the Real Presence of the Body and Blood of Christ in the sacrament of the Eucharist. A few drops of blood began miraculously to ooze from the consecrated Host, thereby confirming what our faith professes.
Urban IV asked one of the greatest theologians of history, St Thomas Aquinas — who at that time was accompanying the Pope in Orvieto — to compose the texts of the liturgical office for this feast. They are masterpieces, in which poetry expresses perfectly the theology. These texts give praise and gratitude to the Most Holy Sacrament, while the mind, penetrating the mystery with wonder, recognizes in the Eucharist the Living and Real Presence of Jesus, of his Sacrifice of love that reconciles us with the Father, and gives us salvation.
The feast of Corpus Christi is one of five occasions in the year on which a diocesan bishop is not to be away from his diocese unless for a grave and urgent reason.
By tradition, Catholics take part in a procession through the streets of a neighborhood near their parish following mass and pray and sing. The Blessed Sacrament is placed in a monstrance and is held aloft by a member of the clergy during the procession. After the procession, parishioners return to the church where benediction takes place. Where the Solemnity of The Most Holy Body and Blood of Christ is not a holy day of obligation, it is assigned to the Sunday after the Most Holy Trinity as its proper day.