Zechariah 8:19 answers the initial question about fasting posed in 7:3. Zechariah says:
Thus says Yahweh of hosts
The fast of the fourth,
And the fast of the fifth,
And the fast of the seventh,
And the fast of the ninth,
Will be to the house of Judah
Truth and peace love!
The verse has a neatly symmetrical organization. Four fasts are mentioned, which commemorated and lamented various stages of Babylon’s conquest of the city of Jerusalem. Zechariah says that these fasts will be turned inside out, and he again uses four phrases to describe the new situation. Global fasting gives way to four-cornered feasting.
The fourth item in the second list, though, stands out. It is a command rather than simply a description of the joy that the remnant of returned exiled will enjoy. It is also not about festivity or gladness. It fits, though, if we keep in mind Isaiah’s teaching concerning the true fast (Isaiah 58). Fasting is going to turn to joy, and also the ritual fasts that Israel has been observing are going to be transformed into the full fasting of truth and peace.
Or, we might say that loving truth and peace is not just the true form of fast, but the true form of feasting. Yahweh will be pleased with Israel’s feasts and celebrations only if Israel’s joy expresses itself in faithfulness and peace.