Judges 11:37-39

International Children’s Bible

³⁷ Then she said, “But let me do one thing. Let me be alone for two months to go to the mountains. I will never marry. So let me and my friends go and cry together.”

³⁸ Jephthah said, “Go.” He sent her away for two months. She and her friends stayed in the mountains. There they cried for her because she would never marry. ³⁹ After two months she returned to her father. Jephthah did to her what he promised to the Lord. Now Jephthah’s daughter had never had a husband.

So this became a custom in Israel.