Attributed to Thomas Celano. See Daniel, Thesaurus Hymnology, Volume II, p. 103. Printed in Missale Romanum. Pavia. (1491). Translation by Abraham Coles. Nolker, monk of St. Gall (about 880) says he saw the lines in a book belonging to the Convent of St. Jumièges. Assigned to Cardinal Frangipani ("Malabrancia"), died, 1294. Also to St. Gregory, St. Bernard, Cardinal Orsini, Agnostino Biella, Humbertus. See Dublin Review, No. 39.https://en.wikiquote.org/wiki/Day