Albert Barnes Commentary Jeremiah 6:4

"Prepare ye war against her; arise, and let us go up at noon. Woe unto us! for the day declineth, for the shadows of the evening are stretched out." — Jeremiah 6:4 (ASV)

Prepare ye war - Rather, Sanctify war against her. War in ancient times was never undertaken without religious solemnities (see Deuteronomy 20:2 note). For some of these, compare to Ezekiel 21:21-23.

At noon - The mid-day heat is so great in the East as to be usually passed under shelter (2 Samuel 4:5; Song of Solomon 1:7). The morning-march of an army was made fasting, and was usually over by eight or nine. But so great is the impatience of the Chaldeans for the assault that they cry, “We will make the assault at noon!”

Woe unto us! - Or,

Alas for us! “for the day” has turned
For the evening shadows are lengthening!