Albert Barnes Commentary Judges 11:40

"that the daughters of Israel went yearly to celebrate the daughter of Jephthah the Gileadite four days in a year." — Judges 11:40 (ASV)

There is no surviving allusion elsewhere to this annual lamentation of the untimely fate of Jephthah’s daughter. But the poetic nature of the narrative suggests that it may be taken from some ancient song (compare the marginal note 4).