Jamieson, Fausset and Brown on 2 Chronicles 20:3-4

3-4. Jehoshaphat ... proclaimed a fast throughout all Judah--Alarmed by the intelligence and conscious of his total inability to repel this host of invaders, Jehoshaphat felt his only refuge was at the horns of the altar. He resolved to employ the aid of his God, and, in conformity with this resolution, he summoned all his subjects to observe a solemn fast at the sanctuary. It was customary with the Hebrew kings to proclaim fasts in perilous circumstances, either in a city, a district, or throughout the entire kingdom, according to the greatness of the emergency. On this occasion, it was a universal fast, which extended to infants (2Ch 20:13; see also Joe 2:15, 16; Jon 3:7).