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 tradition

Read Judges 11:39 Using Other Translations

And it came to pass at the end of two months, that she returned unto her father, who did with her according to his vow which he had vowed: and she knew no man. And it was a custom in Israel,
And at the end of two months, she returned to her father, who did with her according to his vow that he had made. She had never known a man, and it became a custom in Israel
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