O Sacrum Convivium

... is a Latin hymn composed by Thomas Aquinas as an antiphon for the Feast of Corpus Christi, to express the great mystery of the Eucharist:

O sacrum convivium!
    O sacred banquet!
in quo Christus sumitur:
    in which Christ is received,
recolitur memoria passionis eius:
    the memory of his Passion is renewed,
mens impletur gratia:
    the mind is filled with grace,
et futurae gloriae nobis pignus datur.
    and a pledge of future glory to us is given.
Alleluia.

In the traditional form of the Roman Mass, this feast, which occurs on the previous Thursday (the weekday traditionally devoted to the Eucharist), may be celebrated as an "external solemnity" on the following Sunday. This indulgence was granted in the early 20th century to the Dioceses of the United States by the Holy See. In the reformed liturgy, many countries have this feast on a Sunday, and so this is the day that many Eucharistic processions occur, whether around the property of the parish church, or through the streets of town.