וַיִּרָ֕א וַיִּתֵּ֧ן יְהוֹשָׁפָ֛ט אֶת־פָּנָ֖יו לִדְר֣וֹשׁ לַיהוָ֑ה וַיִּקְרָא־צ֖וֹם עַל־כָּל־יְהוּדָֽה׃
And Jehoshaphat feared, and set himself to seek unto the LORD; and he proclaimed a fast throughout all Judah.
Read halakhic rulings (Jewish law) on II Chronicles 20:3: and classical halakhic sources.