John Gill Commentary 2 Chronicles 20:16

"To-morrow go ye down against them: behold, they come up by the ascent of Ziz; and ye shall find them at the end of the valley, before the wilderness of Jeruel." — 2 Chronicles 20:16 (ASV)

Tomorrow go down against them This was the fast day, and so not proper to march out in, but tomorrow they might go out with great confidence and intrepidity; and as Jerusalem was situated on an eminence, they are directed to go down:

behold, they come up by the cliff of Ziz; a steep hill, so called from the flowers upon it:

and you shall find them at the end of the brook; on the bank of Kidron, according to Beckius, which seems not likely to be meant, since they went as far as the wilderness of Tekoa, (2 Chronicles 20:20),

before the wilderness of Jeruel; the same with that of Tekoa, or near it.