Dies Irae (Day of Wrath) is a famous thirteenth century Latin hymn thought to be written by Thomas of Celano. It is a medieval Latin poem characterized by its accentual stress and its rhymed lines. The metre is trochaic. The poem describes the day of judgment, the last trumpet summoning souls before the throne of God, where the saved will be delivered and the unsaved cast into eternal flames.
The hymn is best known from its use as a sequence in the Roman Catholic Requiem Mass. It was removed from the ordinary form of the Roman Rite Mass in the liturgical reform of 1969--1970, but was retained as a hymn of the Divine Office. It can also still be heard when the 1962 form of the Mass is used. An English version of it is found in various missals used in the Anglican Communion.
Those familiar with musical settings of the Requiem Mass—such as those by Mozart or Verdi—will be aware of the important place Dies Iræ held in the liturgy.
It remained as the sequence for the Requiem Mass in the Roman Missal of 1962 (the last edition before the Second Vatican Council) and so is still heard in churches where the Tridentine Latin liturgy is celebrated.