"(Lat., ‘Day of wrath’), the opening words, and hence the name, of the sequence in the Mass for the Dead in the W[estern] Church. It is now thought to go back to a rhymed prayer of late 12th-cent[ury] Benedictine origin. To this prayer, depicting the soul awaiting judgement, a Franciscan . . . has added a greater sense of urgency, reflecting the eschatological mood of the mid-13th cent[ury]. The first printed Missal containing it as the sequence for Requiem Masses is that of Venice, 1485. Until 1969 its use was obligatory. . . . It may now . . . be omitted. . . ."
"Dies Irae" The Oxford Dictionary of the Christian Church. Ed. F. L. Cross and E. A. Livingstone.© Oxford University Press 2005. The Oxford Dictionary of the Christian Church: (e-reference edition). Oxford University Press. Seattle Pacific University. 30 August 2012 http://ezproxy.spu.edu/login?url=http://www.oxford-christianchurch.com/entry?entry=t257.e2035.
The sequence itself, as notated in The new Grove dictionary of music and musicians, 2nd ed., ed. Stanley Sadie (New York: Grove Dictionaries Inc., 2001), s.v. "Dies Irae" (vol. 7, p. 332-333), by John Caldwell and Malcolm Boyd: