John Gill Commentary 2 Chronicles 20:3

"And Jehoshaphat feared, and set himself to seek unto Jehovah; and he proclaimed a fast throughout all Judah." — 2 Chronicles 20:3 (ASV)

And Jehoshaphat feared
Exceedingly, as the Targum adds, not merely because of the number of his enemies, for he had forces enough to go forth against them, see (2 Chronicles 17:14–19) , but chiefly because the Lord had told him that wrath was upon him from him, (2 Chronicles 19:2) and he might fear that this was the time for the execution of it: and set himself to seek the Lord ;
by prayer and supplication, with all seriousness, fervour, and constancy: and proclaimed a fast throughout all Judah ;
a day of humiliation and mourning for sin before the Lord; when they were ordered to abstain from bodily food and labour, that they might be fit and more at leisure for spiritual exercises.