Mishnah Ta'anit 4:8
משנה תענית ד׳:ח׳
Mishnah Ta'anit 4:8
The Mishnah states that from Tu B'Av onward the 'days of the rejoicing of the Temple' begin, pairing Tu B'Av with Yom Kippur as especially joyous dates. It provides the starting point for the custom of increased joy after the fifteenth of Av.
אָמַר רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל, לֹא הָיוּ יָמִים טוֹבִים לְיִשְׂרָאֵל כַּחֲמִשָּׁה עָשָׂר בְּאָב וּכְיוֹם הַכִּפּוּרִים, שֶׁבָּהֶן בְּנוֹת יְרוּשָׁלַיִם יוֹצְאוֹת בִּכְלֵי לָבָן שְׁאוּלִין, שֶׁלֹּא לְבַיֵּשׁ אֶת מִי שֶׁאֵין לוֹ. אַל תִּתֵּן עֵינֶיךָ בַנּוֹי, תֵּן עֵינֶיךָ בַמִּשְׁפָּחָה.
Rabban Shimon ben Gamliel said: There were no days as joyous for the Jewish people as the fifteenth of Av and as Yom Kippur, as on them the daughters of Jerusalem would go out in white clothes, which each woman borrowed from another. And the daughters of Jerusalem would go out and dance in the vineyards.