Albert Barnes Commentary Zechariah 8:19

"Thus saith Jehovah 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 feasts; therefore love truth and peace." — Zechariah 8:19 (ASV)

The fast of the fourth month - On the ninth day of the fourth month of Zedekiah’s eleventh year, Jerusalem, in the extremity of famine, opened to Nebuchadnezzar, and his princes sat in her gate; in the tenth month of his ninth year, Nebuchadnezzar began the siege. Ezekiel was commanded on its tenth day; write thee the name of the day, of this same day, (Ezekiel 24:1–2) as the beginning of God’s uttermost judgments against the bloody city.

The days of national sorrow were to be turned to exuberant joy, joy and gladness and cheerful feasts (Esther 8:17; Esther 9:19; Esther 9:22; Ecclesiastes 7:14), for the sorrows which they commemorated were but the harbingers of joy, when the chastisements were ended.

Only He adds, love the truth and peace; for such love, by which they would be Israelites indeed, in whose spirits is no guile, was the condition for their participation in the blessings of the Gospel, of which he goes on to speak.