The Feast of Corpus Christi is celebrated on this day in some churches, transferred from its true position on the Thursday after Trinity Sunday. It is a day set aside to pay tribute to the Real Presence of Christ, the Body and Blood in the Communion meal.
Saint Juliana of Liege (1193-1258) was the first champion of such a feast day; she wanted a commemoration for the Eucharist that would fall outside the season of Lent and outside Holy Week. She convinced her local bishop, Robert de Thorete, of the usefulness of such an occasion and he declared the first celebration of the feast in 1246. From there it eventually spread to the whole Catholic Church, decreed by Pope Urban in 1264. Other churches, particularly some Anglo-Catholic churches in the Anglican Communion also mark the day. Historically, it was often a very elaborate celebration, with an outdoor procession conveying the elements of the Eucharist.
The well-known Latin text Ave verum corpus dates from the fourteenth century and is attributed to Pope Innocent IV, written for this feast day. It has been set to music by many composers, perhaps most notably by Mozart and William Byrd.
This is a setting I particularly like, by Camille Saint-Saens, sung by the Handel Choir of Baltimore, Maryland.
P.S. - The art above is by the fifteenth-century Venetian painter Giovanni Bellini, depicting a Corpus Christi procession in Venice's St. Mark's Square.
Seven Years Ago: Here, O my God, I see thee face to face
Three Years Ago: Sweet Sacrament divine
