Judges 11: 29-38 (feel free to read the whole chapter if you think i've taken it out of "context")
29 Then the Spirit of the LORD came upon Jephthah. He crossed Gilead and Manasseh, passed through Mizpah of Gilead, and from there he advanced against the Ammonites. 30 And Jephthah made a vow to the LORD : "If you give the Ammonites into my hands, 31 whatever comes out of the door of my house to meet me when I return in triumph from the Ammonites will be the LORD's, and I will sacrifice it as a burnt offering."
32 Then Jephthah went over to fight the Ammonites, and the LORD gave them into his hands. 33 He devastated twenty towns from Aroer to the vicinity of Minnith, as far as Abel Keramim. Thus Israel subdued Ammon.
34 When Jephthah returned to his home in Mizpah, who should come out to meet him but his daughter, dancing to the sound of tambourines! She was an only child. Except for her he had neither son nor daughter. 35 When he saw her, he tore his clothes and cried, "Oh! My daughter! You have made me miserable and wretched, because I have made a vow to the LORD that I cannot break."
36 "My father," she replied, "you have given your word to the LORD. Do to me just as you promised, now that the LORD has avenged you of your enemies, the Ammonites. 37 But grant me this one request," she said. "Give me two months to roam the hills and weep with my friends, because I will never marry."
38 "You may go," he said. And he let her go for two months. She and the girls went into the hills and wept because she would never marry. 39 After the two months, she returned to her father and he did to her as he had vowed. And she was a virgin.
From this comes the Israelite custom 40 that each year the young women of Israel go out for four days to commemorate the daughter of Jephthah the Gileadite.