"Cursed be he who gives his daughter to Benjamin" (Judges 21:18). On the fifteenth of Av, the tribe of Benjamin was permitted to enter the congregation [to intermarry with the other tribes], for it is written, "Cursed be he who gives his daughter to Benjamin" -- they read it as a verse and distanced him, and they read it as a verse and drew him near. They read it as a verse and distanced him: "Cursed be he who gives." They read it as a verse and drew him near: "Ephraim and Manasseh shall be mine like Reuben and Simeon" (Genesis 48:5); they read it as a verse and drew near, for it is written, "a nation and a company of nations shall come from you" (Genesis 35:11) -- and Benjamin had not yet been born [so the promise of tribes must include marrying into Benjamin].
On the Fifteenth of Av Benjamin Was Allowed to Marry In
Yalkut Shimoni on Nach 76:4
ארור נותן בתו לבנימן. בט"ו באב הותרו שבט בנימן לבא בקהל, דכתבי ארור נותן בתו לבנימין, מקרא קראו ורחקו, מקרא קראו וקרבו, מקרא קראו ורחקו, אפרים ומנשה כראובן ושמעון יהיו לי, מקרא קראו וקרבו דכתבי גוי וקהל גוים יהיה ממך ועדין לא נולד בנימן: