In “Preces meae non sunt dignae,” we referred to the Dies irae as “a splendid old hymn.” It has been brought to our attention—by a source we respect very much and have quoted here from time to time—that this is not quite correct. The Dies irae is a sequence historically used in the Requiem. (This is, of course, why your copies of the Mozart and Verdi Requiems have settings of the Dies irae, for example.) It was dropped from its venerable position in the Mass in the Bugnini revisions, though, which is why it got transported over to the Liturgia Horarum as an optional hymn for the thirty-fourth week of Tempus Per Annum, according to the same source.
We regret the error, not least on account of who pointed it out.