After the victory over the Ammonites, Jephthah returned home to Mizpah. As he arrived at his house, his daughter came out to meet him with timbrels and dancing. She was his only child. When he saw her, he tore his clothes because of his vow. She told him that she understood, and asked for two months to go and wander the mountains with her friends. He let her go, and when she returned to her father, he carried out his vow, and she knew no man. After that, it became a custom that the daughters of Israel went four days each year to lament the daughter of Jephthah the Gileadite.
This story gives us two very clear lessons. First, making a vow before the Lord or in any setting, it is important to not make hasty or rash decisions. God desires to bless us and give us victory, regardless of any vow that may be proclaimed. Second, if you do make a vow, it is important to fulfill that vow, and to be a man or woman of your word. It is wrong to make a vow and then disregard that vow.