כֹּֽה־אָמַ֞ר יְהוָ֣ה צְבָא֗וֹת צ֣וֹם הָרְבִיעִ֡י וְצ֣וֹם הַחֲמִישִׁי֩ וְצ֨וֹם הַשְּׁבִיעִ֜י וְצ֣וֹם הָעֲשִׂירִ֗י יִהְיֶ֤ה לְבֵית־יְהוּדָה֙ לְשָׂשׂ֣וֹן וּלְשִׂמְחָ֔ה וּֽלְמֹעֲדִ֖ים טוֹבִ֑ים וְהָאֱמֶ֥ת וְהַשָּׁל֖וֹם אֱהָֽבוּ׃ (פ)
’Thus saith the LORD 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, shall be to the house of Judah joy and gladness, and cheerful seasons; therefore love ye truth and peace.
Rashi on Zechariah
the fast of the fourth [month] The fast of Tammuz, which is the fourth of the months.
Radak on Zechariah
The fast of the fourth — This is Tammuz, when they fasted in exile on the seventeenth of the month, the day that the city was broken into.