Midrash su Giobbe 30:31

Rabbi Ze’eira began: “My lyre is for mourning, and my flute is for the voice of weepers” (Job 30:31). There it is taught: Rabban Shimon ben Gamliel said: There were no days as joyous for Israel as the fifteenth of Av and as Yom Kippur, on which the daughters of Jerusalem would go out in white borrowed garments so as not to embarrass one who did not have one. All the garments require immersion. The daughters of Israel would go out in them and dance in the vineyards.¹²⁷Mishna Taanit 4:8. It is taught: One who did not have a wife would turn to there. What would they say? ‘Young man, lift your eyes and see what you are choosing for yourself. Do not look at beauty, look at lineage.’ Likewise it says: “Emerge, daughters of Zion, and gaze at King Solomon, at the crown with which his mother crowned him on the day of his wedding, and on the day of the rejoicing of his heart” (Song of Songs 3:11). “The day of his wedding” – this is the giving of the Torah. “The day of the rejoicing of his heart” – this is the building of the Temple, may it be built speedily in our days.
Granted Yom Kippur, as it is a day of pardon and forgiveness for Israel, the day on which the last tablets were given. However, what is the fifteenth of Av? Rabbi Yaakov bar Aḥa said in the name of Rabbi Asi: It is the ideal time for chopping trees, as all the trees chopped on it do not produce a worm, and it is taught: Any wood in which a worm is found is disqualified from being atop the altar.¹²⁸Mishna Midot 2:5. Rabbi Abba bar Kahana and Rabbi Asi said in the name of Ulla in the name of Rabbi [Yehuda HaNasi]: It was then that Hoshea ben Ela canceled the sentries that Yerovam ben Nevat had deployed on the roads.¹²⁹Yerovam, king of Israel, appointed sentries to prevent the residents of his kingdom from ascending to Jerusalem for the pilgrimage festivals. Rav Kahana asked before Rav: Is it possible that he did all this good and it is written in his regard: “Shalmaneser, king of Assyria, ascended against him, [and Hoshea became his servant]” (II Kings 17:3)? Rather, it is because he removed the collar from his neck and placed it on the neck of the masses, and he did not say: All the people should ascend and pray, but rather, anyone who wishes to ascend let him ascend.¹³⁰Hoshea was punished, and the rest of his kingdom was punished as well (see II Kings 17:6), because he did not encourage them to ascend to Jerusalem and most of the people did not actually ascend. Previously it had been the king’s fault that the people did not ascend, as they could not go because of the sentries; subsequently it was the people’s fault, and therefore the entire kingdom was punished.
Rabbi Shmuel bat Naḥmani [said], and some say it in the name of Rabbi Shmuel bar Yitzḥak: [The fifteenth of Av is] the day the tribes were permitted to enter into marriage with one another, as it is stated: “And every daughter who inherits an inheritance [from the tribes of the children of Israel shall be a wife to one of the family of the tribe of her father]” (Numbers 36:8), and it is written: “And no inheritance shall pass from tribe to another tribe…” (Numbers 36:9). Is it possible for a daughter to inherit [from] two tribes? Rather, say on this basis, her father was from one tribe and her mother from another tribe.¹³¹The verse refers to a daughter who inherits “from the tribes of the children of Israel,” implying that her parents were from different tribes. But then the verse goes on to state that a woman cannot marry a man from a different tribe. The Sages understood this to mean that it was only in the first generation when Israel entered the land that women could not marry men from other tribes. The date when it was officially determined that from then on it was permissible was the fifteenth of Av.
The Rabbis said: [It is] the day the tribe of Benjamin was permitted to enter the congregation, as it is written: “Cursed is one who gives a woman to Benjamin” (Judges 21:18). Rabbi Yoḥanan said: They read a verse and drew them near, they read a verse and distanced them. They read a verse and drew them near: “A nation and an assembly of nations shall be from you” (Genesis 35:11).¹³²When God promised this to Jacob, Benjamin had not yet been born, meaning that it was necessary to ensure that Benjamin would procreate. They read a verse and distanced them: “Ephraim and Manasseh will be like Reuben and Simeon for me” (Genesis 48:5) – as they are not considered with their brothers.¹³³The total of twelve tribes could be achieved without Benjamin, with the addition of Ephraim and Manasseh. Rav Yehuda said that Shmuel said: It was the day that the tribes were permitted [to marry each other].
Rav Matna said: It was the day that the slain of Beitar were allowed to be buried. Rabbi Eliezer the Great said: It is reasonable on the fifteenth; from that point on, the intensity of the sun wanes and they would no longer chop wood for the arrangement. Rabbi Menasya said: They called it the day of the breaking of the scythe. From that point on: One who adds, adds, and one who does not add, will be gathered.¹³⁴From this point on, the nights begin to be lengthy. Since night is a good time for Torah study, the midrash states that years of life will be added for one who adds to his hours of Torah study. One who does not will die young.
Rabbi Avin and Rabbi Yoḥanan said: It is the day that the digging for those who died in the wilderness was halted. Rabbi Levi said: Every eve of the ninth of Av, Moses would dispatch a herald to the entire camp, saying: ‘Go out and dig,’ and they would go out and dig graves and sleep in them. In the morning, he would dispatch a herald saying: ‘Rise and separate the dead from the living,’ and they would stand and take themselves out. Fifteen thousand and more were subtracted,¹³⁵Each year for a total of six hundred thousand. In the fortieth year, the last one, they did so and found themselves intact. They said: It appears that we were mistaken in our calculation, and they did the same on the tenth, the eleventh, the twelfth, the thirteenth, and the fourteenth. When the moon was full, they said: It appears that the Holy One blessed be He abrogated the decree from upon us, and they then rendered it a holiday. But due to their iniquities, mourning beset this world with the destruction of the Temple twice. That is what is written: “My lyre is for mourning, and my flute is for the voice of weepers” (Job 30:31). “The people wept that night” (Numbers 14:1) – when they were exiled, Jeremiah began lamenting over them: “How does…sit solitary?” (Lamentations 1:1).