Komentarz do Diwrej ha-jamim II 35:25

And all the male...singers - הַשָּׁרִים, an expression of song, choristers, for both song and lamentation are recited with the same voice. Concerning him, Amos prophesied (8:9): “and I will cause the sun to set at midday, and I will darken the land on a sunny day.” This is the day of Josiah, who died at the age of thirty-nine, in the middle of his days, for it is written (Ps. 90:10): “The days of our years because of them are seventy years,” and it is written (Amos 8:10): “And I will turn your festivals into mourning, and all your songs into lamentation, etc. and I will make it like the mourning for an only son, and its end like a bitter day.” This is what [the Chronicler] says: “And all the male and female singers said in their lamentations,” “and all of Judah and Jerusalem were mourning” for him.

and they made them a statute When any grief or weeping befalls them, for which they lament and weep, they mention this grief with it. An example is the Ninth of Av, in which we recite lamentations for those slain in the persecutions that occurred in our times. They will similarly bewail the day of Josiah’s death. An example is (Jud. 11:39f.): “and it became a statute in Israel, etc. to lament the daughter of Jephthah the Gileadite, four days in a year.”

and behold, they are written in the lamentations in the Book of Lamentations.