Albert Barnes 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)

General fasts had been previously observed by the Israelites (for example, Judges 20:26; 1 Samuel 7:6), but we do not hear of any fast having been “proclaimed” by authority before this.