Solemnity of the Most Holy Body and Blood of Christ
The Feast of Corpus Christi is the feast celebrating the institution of the Holy Eucharist. Maundy Thursday would seem to be the best day to celebrate the institution of the Eucharist, because that is the day it was actually instituted. However, the emphasis on the passion themes present in the Maundy Thursday celebration created the need for another day to focus entirely on the institution of the Mass.
The Thursday after Trinity Sunday was chosen because it is a Thursday (the same day Christ instituted the Eucharist) and it is the first free Thursday after the Easter season. Thus Corpus Christi falls within the Season of Ordinary Time. Typically Corpus Christi services consist of singing traditional hymns, Lauda Sion and Pange Lingua, both attributed to St. Thomas Aquinas. St. John Cantius typically holds an outdoor procession of the Blessed Sacrament as a way to celebrate Corpus Christi. Eucharistic Exposition and Benediction follows.
In some ways every Sunday is a feast of the Institution of the Eucharist, because in receiving Communion, we are recalling its institution. The feast of Corpus Christi owes a rather large debt to Juliana, a nun of Liege who was led to start a celebration of the Mass around AD 1230. In AD 1264 a bull of Pope Urban IV commanded the observance of the feast. By the 14th century, the feast became universally celebrated in the West. St. Thomas Aquinas is given credit for many of the customs and hymns associated with Corpus Christi.