Who's going to lead Israel this time? We're so glad you asked…
There was this guy in Israel named Gilead, and he had a son with a harlot and named him Jephthah.
When Jephthah grew up, his half-brothers ran him out of town so he wouldn't take any of their inheritance because, after all, he was "the son of a strange woman [a.k.a. a harlot]" (KJV 11:2).
Years later, the Ammonites are about to attack Israel, and because Jephthah is "a mighty man of valour" (KJV 11:1), his brothers ask him to come be their captain.
But Jephthah's like, "Oh, now you want to be friends?" And they're all, "Please? You'll be our leader after it's all through." So Jephthah agrees, and becomes the leader of Israel's army.
Jephthah sends a messenger to see the king of the Ammonites and ask why he's attacking.
The king responds, "Because Israel stole my land."
Jephthah says, "Nuh-uh. God gave us this land. It wasn't even yours to begin with."
The king says, "I don't think so, buckaroo. Prepare for war."
"Then the Spirit of the Lord came upon Jephthah, and he […] vowed a vow unto the Lord, and said, If thou shalt without fail deliver the children of Ammon into mine hands, Then it shall be, that whatsoever cometh forth of the doors of my house to meet me, when I return in peace from the children of Ammon, shall surely be the Lord's, and I will offer it up for a burnt offering" (KJV 11:29-31). Promises, promises, Jeph…
The Lord does deliver the Ammonites into Jephthah's hands, and he "smote them […] with a very great slaughter. And the children of Ammon were subdued before the children of Israel" (KJV 11:33). Guess that settles that.
So Jephthah heads home, triumphant. He remembers the vow he made to God, so he's watching for some kind of animal to come running (a goat, a lamb, a dog, an armadillo, etc.).
But to his horror, as he approaches his house, who should come out to meet him—with timbrels (like tambourines) and dances, no less—but his daughter (who's an only child, by the way).
She takes the news of her father's promise to God surprisingly well, and agrees to submit. She only asks that he give her two months to "go up and down upon the mountains, and bewail my virginity, I and my fellows" (KJV 11:37).
What father could deny his daughter one last virginity pity-party with her friends?
After her two months are up, Jephthah's daughter returns, and he offers her as a sacrifice to God.
The daughters of Israel commemorate this sad, sad event each year by lamenting Jephthah's daughter for four days.