Lives of the Saints, by Alban Butler, Benziger Bros. ed. [1894], at sacred-texts.com
ST. MAMMERTUS, Archbishop of Vienne in Dauphin, was a prelate renowned for his sanctity, learning, and miracles. He instituted in his diocese the fasts and supplications called the Rogations, on the following occasions. Almighty God, to punish the sins of the people, visited them with wars and other public calamities, and awaked them from their spiritual lethargy by the terrors of earthquakes, fires, and ravenous wild beasts, which last were sometimes seen in the very market-place of cities. These evils the impious ascribed to blind chance; but religious and prudent persons considered them as tokens of the divine anger, which threatened their entire destruction. Amidst these scourges, St. Mammertus received a token of the divine mercy. A terrible fire happened in the city of Vienne, which baffled the efforts of men; but by the prayers of the good bishop the fire on a sudden went out. This miracle strongly affected the minds of the people. The holy prelate took this opportunity to make them sensible of the necessity and efficacy of devout prayer, and formed a pious design of instituting an annual fast and supplication of three days, in which all the faithful should join, with sincere compunction of heart, to appease the divine indignation by fasting, prayer, tears, and the confession of sins. The Church of Auvergne, of which St. Sidonius was bishop, adopted this pious institution before the year 475, and it became in a very short time a universal practice. St. Mammertus died about the year 477.
Reflection.—"Know ye that the Lord will hear your prayers, if you continue with perseverance in fastings and prayers in the sight of the Lord" (Judith iv. 11).