Apparebit repentina dies magna Domini. Anon. cir. 7th century. [Advent.] The earliest reference which we have to this hymn is in Bede's De Metris (672-735). It is an acrostic, the first verse commencing with A, the third with B, the fifth with C, &c. Dr. Neale speaks of it as a "rugged, but grand Judgment Hymn," dates it "as early as the 7th century," and declares that "it manifestly contains the germ of the Dies Irae." The text is given in Cassander's Hymni Ecclesiastici, Col. 1556; Thomasius, vol. ii. p. 433; Rambach, Anthologie, i. p. 126; Daniel, 1841, vol. i. No. 161; Du Méril, Poésies Populaires Latines, 1843, p. 135; Trench's S. Latin Poetry, 1849 and 1873, and others. [Rev.W. A. Shouts, B. D.]
Translation in common use:—
1. That great day of wrath and terror. By J. M. Neale, in his Mediaeval Hymns, 1851, p. 9. From this translation a cento has been given in the Cumbrae Hymn Book, 1863. No. 235. Mrs. Charles has also rendered it as: "Suddenly to all appearing the great day of God shall come," in her Voice of Christian Life in Song, 1858, p. 142, but it is not in common use.
-- John Julian, Dictionary of Hymnology (1907)