O sacrum convivium is the Antiphon upon Magnificat at Second Vespers of Corpus Christi; and is a hymn in praise of the Blessed Sacrament. The Gregorian melody is very beautiful (see the 2nd video below), and the text has been set by many composers; this version is by Tomas Luis de Victoria (1548-1611):
O sacrum convivium!Here, the Belgian chant group Psallentes sings the Gregorian antiphon, and they include this note: "The antiphon O sacrum convivium, from a sixteenth century Turnhout processionale (Flanders, ca. 1550)."
in quo Christus sumitur:
recolitur memoria passionis ejus:
mens impletur gratia:
et futurae gloriae nobis pignus datur.
O sacred banquet!
in which Christ is received,
the memory of his Passion is renewed,
the mind is filled with grace,
and a pledge of future glory to us is given.