Albert Barnes Commentary 2 Chronicles 35:25

"And Jeremiah lamented for Josiah: and all the singing men and singing women spake of Josiah in their lamentations unto this day; and they made them an ordinance in Israel: and, behold, they are written in the lamentations." — 2 Chronicles 35:25 (ASV)

Some find Jeremiah’s lament in the entire Book of Lamentations; others in a part of it, Lamentations 4. However, most critics are of the opinion that the lament is lost. Days of calamity were commemorated by laments on their anniversaries, and this was one of them. The “Book of Dirges” was a collection of such poems that once existed but is now lost.

And made them an ordinance — Rather, “and they made them an ordinance,” that is, those who had authority to do so, not the minstrels.