Adam Clarke on Judges 20:26

And wept - Had they humbled themselves, fasted, and prayed, and offered sacrifices at first, they had not been discomfited.

And fasted that day until even - This is the first place where fasting is mentioned as a religious ceremony, or as a means of obtaining help from God. And in this case, and many since, it has been powerfully effectual. At present it is but little used; a strong proof that self-denial is wearing out of fashion.