Judges 11:37-39

37She also said to her father, “Let me do this one thing: Let me wander for two months through the mountains with my friends and mourn my virginity.”

38“Go,” he said. And he sent her away for two months.

So she left with her friends and mourned her virginity upon the mountains. 39After two months, she returned to her father, and he did to her as he had vowed. And she had never had relations with a man.

So it has become a custom in Israel