John Gill 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)

And Jeremiah lamented for Josiah
Composed a lamentation for him, which is now lost; for what is said in (Lamentations 4:20) respects Zedekiah, and not Josiah:

and all the singing men, and all the singing women, spoke of Josiah in
their lamentations unto this day ;
who were made use of on mournful occasions, as the "preficae" among the Romans, see (Jeremiah 9:17) these in their mournful ditties used to make mention of his name, and the disaster that befell him:

and made them an ordinance in Israel ;
an annual constitution, as the Targum calls it, appointing a solemn mourning for him once a year, which Jarchi says was on the ninth of Ab or July:

and, behold, they are written in the lamentations ;
not of Jeremiah; though the Targum is, lo, they are written in the book which Baruch wrote from the mouth of Jeremiah, concerning the lamentations, but respect a collection of lamentations on various subjects then in being, but since lost.