Charles Ellicott Commentary Zechariah 7:5

"Speak unto all the people of the land, and to the priests, saying, When ye fasted and mourned in the fifth and in the seventh [month], even these seventy years, did ye at all fast unto me, even to me?" — Zechariah 7:5 (ASV)

All the people.—The question, though asked by only a few, was of interest to all the people; or the people of Bethel may have been the representatives of all the people; in any case, the reply is given to the whole nation (Zechariah 7:5). Though the mission came in the ninth month, no question was asked about the fast of the tenth month, but only about that of the fifth month. The reason for this appears to be that, the fast in Ab being connected with their mourning for the destruction of the Temple, it was natural that, now its rebuilding had progressed so far, they should inquire whether that particular fast should be kept.

The prophet, in his first reply, also mentions the fast of the 3rd of the seventh month (Tishri), which was kept in memory of the assassination of Gedaliah, which took place soon after the destruction of the Temple. The seventy years to which he refers are those between the seventh month B.C. 587 (the date of the assassination of Gedaliah) and the ninth month B.C. 518 (the date of the Bethel mission).