Jephthah's daughter was the judge's only child.
While at war with the Ammonites, the Jephthah brashly promised God to sacrifice the first thing that comes out of his house to greet him if God were to grant him and the Israelites victory.
If You will indeed give the Ammonites into my hand, then whatever comes out of the doors of my house to meet me when I return in peace from the Ammonites, it shall be the Lord’s, and I will offer it up as a burnt offering.— Judges 11:30–11:31 (AMP)
God granted Jephthah victory, and Jephthah returned home. When he arrived, his daughter came out and greeted him. Jephthah realized that his daughter was the prize of the vow he made and explained the situation to his daughter.
Alas, my daughter! You have brought me great disaster, and you are the cause of ruin to me; for I have made a vow to the Lord, and I cannot take it back.— Judges 11:35 (AMP)
Jephthah's daughter accepted her fate.
My father, you have made a vow to the Lord; do to me as you have vowed, since the Lord has taken vengeance for you on your enemies, the Ammonites.— Judges 11:36 (AMP)
And asked for two months to mourn over her unfulfilled (i.e. unmarried) life with her friends.
Let this one thing be done for me; let me alone for two months, so that I may go to the mountains and weep over my virginity, I and my companions.— Judges 11:37 (AMP)
Jephthah granted his daughter's requests, and after two months of mourning, his daughter returned to him in order to be sacrificed, and Jephthah sacrificed his daughter.