Jephthah was made the ruler and commander of Israel. Here are two important details in his life ~ he had one child, a daughter, and he had a vow to God. He wanted victory against the Ammonites so he made the vow to God.
29 At that time the Spirit of the Lord came upon Jephthah, and he went throughout the land of Gilead and Manasseh, including Mizpah in Gilead, and from there he led an army against the Ammonites. 30 And Jephthah made a vow to the Lord. He said, “If you give me victory over the Ammonites, 31 I will give to the Lord whatever comes out of my house to meet me when I return in triumph. I will sacrifice it as a burnt offering.”
32 So Jephthah led his army against the Ammonites, and the Lord gave him victory. 33 He crushed the Ammonites, devastating about twenty towns from Aroer to an area near Minnith and as far away as Abel-keramim. In this way Israel defeated the Ammonites.
34 When Jephthah returned home to Mizpah, his daughter came out to meet him, playing on a tambourine and dancing for joy. She was his one and only child; he had no other sons or daughters. 35 When he saw her, he tore his clothes in anguish. “Oh, my daughter!” he cried out. “You have completely destroyed me! You’ve brought disaster on me! For I have made a vow to the Lord, and I cannot take it back.”
36 And she said, “Father, if you have made a vow to the Lord, you must do to me what you have vowed, for the Lord has given you a great victory over your enemies, the Ammonites. 37 But first let me do this one thing: Let me go up and roam in the hills and weep with my friends for two months, because I will die a virgin.”
38 “You may go,” Jephthah said. And he sent her away for two months. She and her friends went into the hills and wept because she would never have children. 39 When she returned home, her father kept the vow he had made, and she died a virgin.
So it has become a custom in Israel 40 for young Israelite women to go away for four days each year to lament the fate of Jephthah’s daughter.