From the Wikipedia entry:
Corpus Christi (Latin for Body of Christ) is a Christian feast in honour of the Holy Eucharist. It was originally assigned to the Thursday following Trinity Sunday, thereby mirroring Holy Thursday, the Thursday of Holy Week, the day on which Christians commemorate The Last Supper of Jesus Christ and his apostles, seen as the first Holy Eucharist. From 2007 the Roman Catholic Church in England and Wales celebrates the Feast of Corpus Christi on the Sunday after Trinity Sunday – on the Sunday after the traditional Thursday celebration in other countries.
The appearance of Corpus Christi as a feast in the Christian calendar was primarily due to the petitions of the thirteenth-century Augustinian nun Juliana of Liège. From her youth she claimed that God had been instructing her to establish a feast day for the Eucharist and later in life petitioned the learned Dominican Hugh of St-Cher, Jacques Pantaléon (Archdeacon of Liège and later Pope Urban IV) and Robert de Thorete, Bishop of Liège. At that time bishops could order feasts in their dioceses, so in 1246 Bishop Robert convened a synod and ordered a celebration of Corpus Christi to be held each year thereafter. The decree is preserved in Anton Joseph Binterim’s Vorzüglichsten Denkwürdigkeiten der Christkatholischen Kirche, together with parts of the first liturgy written for the occasion.
The celebration of Corpus Christi only became widespread after both Juliana and Bishop Robert had died. In 1263, Jacques Pantaléon, now Pope Urban IV, investigated claims of a miracle in which blood had issued from a host. One alternate theory is that the blood was actually a clustering of Serratia marcescens, a reddish bacteria that often grows on bread. Regardless, in 1264 he issued the papal bull Transiturus in which Corpus Christi was made a feast day. A new liturgy for the celebration was written by Thomas Aquinas.
While the institution of the Eucharist is celebrated on Holy Thursday, the joy of what is referred to in Greek as “the Holy Gift” (τὸ Ἅγιον Δῶρον) cannot on that occasion be well expressed, because of the nearness of Good Friday. This is given as a reason for celebrating the Corpus Christi feast at a different time of year.
In other news, I just discovered that the US Navy named one of their nuclear-powered submarines the USS Corpus Christi. Irony, anyone? No? Blasphemy then?