Halakhah su Zaccaria 8:19

And these four fasts; behold they are explicit in the tradition through the prophet, Zachariah 8:19, "So said 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 will be for the House of Yehuda for happiness and rejoicing.'" "The fast of the fourth" is the seventeenth of Tammuz, as it is in the fourth month. And "the fast of the fifth" is the ninth of Av, as it is in the fifth month. And "the fast of the seventh" is the Fast of Gedaliah, as it is in the seventh of the months. And "the fast of the tenth" is the tenth of Tevet, as it is in the tenth of the months. And all of Israel is accustomed to fast on those days. And even though with the first Temple, Jerusalem was breached on the ninth of Tammuz - as it is written in Kings there - nonetheless, they did not decree a fast upon it; only on the seventeenth, upon which the city was breached with the second [Temple]. And for us the destruction of the second Temple is [more] weighty, as from then we have become poor and scattered to the four corners of the earth. And to decree that they should fast on [both] the ninth and the seventeenth, we do not decree too much upon the community. (And Beit Yosef brought in the name of Rabbenu Yerucham that Gedaliah was killed on Rosh Hashanah, and it was pushed off to the next day - see there. And nonetheless, it does not have the status of a fast that is pushed off, concerning one who needs to do a circumcision - as it is like this all of the years. And it is not similar to a fast that is pushed off from Shabbat to Sunday, as everyone [is aware] of the pushing off, which is not the case with the Fast of Gedaliah, as all years are the same. And [this is] like the conclusion of Taz - see there.)