Jamieson, Fausset and Brown on Zechariah 7:3

3. Should I weep in the fifth month--"I" represents here the people of God (compare Zec 8:21). This rather favors Maurer's view, taking "the house of God," the congregation, as nominative to "sent." Their hypocrisy appeared because they showed more concern about a ceremony of human institution (not improper in itself) than about moral obedience. If, too, they had trusted God's promise as to the restoration of Church and State, the fast would have now given place to joy, for which there was more cause than for grief [Pembellus].

to the prophets--Haggai and Zechariah especially. The tenth day of the fifth month was kept a fast, being the anniversary of the destruction of Jerusalem (Jr 52:12-14). They ask, Should the fast be continued, now that the temple and city are being restored?

separating myself--sanctifying myself by separation, not only from food, but from all defilements (compare Joe 2:16) as was usual in a solemn fast.