The feast of Corpus Christi, also called Solemnity of the Most Holy Body and Blood of Christ, is festival in honour of the real presence of the body (corpus) of Jesus Christ in the Eucharist. A movable observance, it is observed on the Thursday (or, in some countries, the Sunday) after Trinity Sunday and is a holy day of obligation in many countries.
The Feast of Corpus Christi originated in 1246 when the Bishop of Liège, ordered the festival to be celebrated in his diocese, however, it did not become wide-spread until 1264 when Pope Urban IV ordered the whole church to observe the feast. By the 15th century it became one of the principal feasts of the church.
While the institution of the Eucharist is celebrated on Holy (Maundy) Thursday, the liturgy on that day also commemorates Christ's washing of the disciples' feet, the institution of the priesthood and the agony in the Garden of Gethsemane. So many other functions took place on this day that the principal event was almost lost sight of. This is mentioned in the Bull Transiturus as the chief reason for the introduction of the new feast. Hence, the feast of Corpus Christi was established to create a feast focused solely on the Holy Eucharist.
A prominent feature of the day is the procession of the Blessed Sacrament, followed by Benediction. It is traditional to decorate the streets with flowers depicting the Holy Eucharist, and to sing devotional hymns in procession. In mediæval times, Corpus Christi was the time for performances of “mystery” plays.