Thomas of Celano was born at Celano in the Abruzzi, and joined St. Francis of Assisi c. 1214. He was commissioned by Gregory IX to write the life of St. Francis: the First Legend, 1229; the Second Legend, 1247; and the Tract on the Miracle of St. Francis a few years later. His Legend of St. Clare was composed in 1255. He was probably among the first band of friars to visit Germany, 1221.
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Dies irae, dies illa, pp. 295-301. In a manuscript in the British Museum of the beginning of the 14th century (Harl. 2888, f. 170 b), there is a Responsory, which might possibly have suggested at least some of the allusions in the "Dies irae." The first part reads:—
"Libera me Domine, de morte aeterna,
in die iliatremenda;
Quando cocli movendi sunt et terra,
Dum veneris judicare saeculum per ignem.
Dies ilia, dies irae, calamitatis et miseriae, dies
inagna et amara valde.
Quid ergo miserrimus, quid dicam vel quid factam,
dum nil boni perferam ante tantum judicem."
This Responsory is also in a British Museum manuscript of the beginning of the 13th cent. (Lansdowne, 431,f. 122 b), and is evidently referred to in a 12th century Life of Gundulf, Bp. of Rochester, who died 1108. See Henry Wharton's Anglia Sacra, vol. ii., 1691, p. 286. He prints the Life from a manuscript now in the British Museum (Nero, A. viii.). The various texts of the "Dies irae," with a full commentary, are in Dr. J. Kayser's Beiträge (p. 655, ii.), ii., 1886, pp. 193-235.
--John Julian, Dictionary of Hymnology, Appendix, Part II (1907)