Midrash su Zaccaria 8:19

GEMARA: Why were they not also sent out for Tamuz and Tebheth [for the fasts]? (Ib. b) Did not R. Ghana b. Bizna say, in the name of R. Simon, the pious: "What is the meaning of the passage (Zach. 8, 19) Thus saith the Lord of hosts; the fast of the fourth, and the fast of the fifth, and the fast of the seventh, and the fast of the tenth shall become in the house of Judah joy and gladness, etc.; i.e., they are called fasts, and also days of joy and gladness! We must therefore say that in peaceful times they shall be for joy and gladness, but, when there is no peace, they shall be fasts." [Hence they were always observed] "This is the meaning," said R. Papa, "when there is peace, these days should be for joy and gladness; in the time of persecution they shall be fasts; at times when there are neither persecutions nor peace, people may or may not fast, as they see fit." If that is so, why then [should messengers have been sent out] on account of the fast of Ab (during peaceful times)? "The fast [ninth day] of Ab is different," explained R. Papa, "since many misfortunes occurred on that day; as the master said: 'On the ninth of Ab, the first and second Temples were destroyed, Bether was captured, and the city of Jerusalem was razed to the ground.' "

(20) MISHNAH: For the proclamation of six New Moon days, messengers are sent out: for Nissan, on account of the Passover; for Ab, on account of the fast (ninth of Ab); for Elul, on account of New Year's; for Tishri, in order to arrange the dates of the [remaining] festivals; for Kislev, on account of Chanukah (the Feast of Maccabees); for Adar, on account of the Feast of Purim (the Feast of Esther). (21) GEMARA: Why were they not also sent out for Tamuz and Tebheth [for the fasts]? (Ib. b) Did not R. Ghana b. Bizna say, in the name of R. Simon, the pious: "What is the meaning of the passage (Zach. 8, 19) Thus saith the Lord of hosts; the fast of the fourth, and the fast of the fifth, and the fast of the seventh, and the fast of the tenth shall become in the house of Judah joy and gladness, etc.; i.e., they are called fasts, and also days of joy and gladness! We must therefore say that in peaceful times they shall be for joy and gladness, but, when there is no peace, they shall be fasts." [Hence they were always observed] "This is the meaning," said R. Papa, "when there is peace, these days should be for joy and gladness; in the time of persecution they shall be fasts; at times when there are neither persecutions nor peace, people may or may not fast, as they see fit." If that is so, why then [should messengers have been sent out] on account of the fast of Ab (during peaceful times)? "The fast [ninth day] of Ab is different," explained R. Papa, "since many misfortunes occurred on that day; as the master said: 'On the ninth of Ab, the first and second Temples were destroyed, Bether was captured, and the city of Jerusalem was razed to the ground.' "

Similarly (Zechariah 8:19) "Thus has said the L-rd of hosts: The fast of the fourth (month), and the fast of the fifth, and the fast of the seventh, and the fast of the tenth will be for the house of Judah for joy and for gladness and for goodly festivals": "the fast of the fourth" — the seventeenth of Tammuz when the city was breached. "the fast of the fifth" — the ninth day of Av, when both the first and second temples were destroyed. "the fast of the seventh" — the third of Tishrei, when Gedaliah ben Achikam was killed. And who killed him? Yishmael ben Netanyah — to teach that the death of the righteous is as grievous to the Holy One Blessed be He as the destruction of the Temple. "the fast of the tenth" — the tenth of Teveth, when the king of Bavel came to besiege Jerusalem, as it is written (Ezekiel 24:1-2) "And the word of the L-rd came to me in the ninth year, saying: Son of man, write for yourself the name of the day, this very day. The king of Bavel has come against Jerusalem (to besiege it) this very day." (Thus, R. Akiva). And I say: "the fast of the tenth" — the fifth day of Teveth, when the report came to the exile that the city had been smitten, as it is written (Ibid. 33:21) "And it was, in the twelfth year of the month on the fifth of the month that a fugitive came to me from Jerusalem, saying 'The city has been conquered.'" And when they heard, they made the day of the hearing (a fast) like the day of the burning. And I find my view more cogent than that of R. Akiva. [For I speak of first things (in the verse) first, and of last things, last, and he speaks of first things last, and of last things first. For he counts according to the order of the months, and I count according to the order of the occurrences. And in Judah they fast for the act, and in the Galil, for the report.]