Taanit 30b
תענית ל׳ ב — ד"ה אָמַר רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל
Taanit 30b:8
Rabban Shimon ben Gamliel teaches that the fifteenth of Av and Yom Kippur are the two happiest days for the Jewish people; the Gemara explains that the fifteenth of Av commemorates the day when the prohibition against intermarriage between tribes was lifted, allowing each tribe to marry into another tribe in order to preserve tribal land inheritance.
אָמַר רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל: לֹא הָיוּ יָמִים טוֹבִים לְיִשְׂרָאֵל כַּחֲמִשָּׁה עָשָׂר בְּאָב וּכְיוֹם הַכִּפּוּרִים. בִּשְׁלָמָא יוֹם הַכִּפּוּרִים — מִשּׁוּם דְּאִית בֵּיהּ סְלִיחָה וּמְחִילָה, יוֹם שֶׁנִּיתְּנוּ בּוֹ לוּחוֹת הָאַחֲרוֹנוֹת. אֶלָּא חֲמִשָּׁה עָשָׂר בְּאָב מַאי הִיא? אָמַר רַב יְהוּדָה אָמַר שְׁמוּאֵל: יוֹם שֶׁהוּתְּרוּ שְׁבָטִים לָבוֹא זֶה בָּזֶה.
§ The mishna taught that Rabban Shimon ben Gamliel said: There were no days as happy for the Jewish people as the fifteenth of Av and as Yom Kippur. The Gemara asks: Granted, Yom Kippur is a day of joy because it has the elements of pardon and forgiveness, and moreover, it is the day on which the last pair of tablets were given. However, what is the special joy of the fifteenth of Av? Rav Yehuda said that Shmuel said: This was the day on which the members of different tribes were permitted to enter one another’s tribe, by intermarriage. It was initially prohibited to intermarry between tribes, so as to keep each plot of land within the portion of the tribe that originally inherited it. This halakha was instituted by the Torah in the wake of a complaint by the relatives of the daughters of Zelophehad, who were worried that if these women married men from other tribes, the inheritance of Zelophehad would be lost from his tribe (see Numbers 36:1–12).