Times for Fasting

In general we must hold that whenever any religious controversy arises, which either a council or ecclesiastical tribunal behooves to decide; whenever a minister is to be chosen; whenever, in short any matter of difficulty and great importance is under consideration: on the other hand, when manifestations of the divine anger appear, as pestilence, war, and famine, the sacred and salutary custom of all ages has been for pastors to exhort the people to public fasting and extraordinary prayer.

Calvin, Institutes, IV, 12, 14