Should We Fast On Tisha B'Av?  
Zechariah 7:3

Why do we fast on Tisha B'Av (the 9th day of the month of Av)?  The primary answer, of course, is to remember the destruction of the Temple and Jerusalem.  Yet, according to Zechariah (chapters 7 and 8), not only Tisha B'Av, but all four of the fast days in which we remember Jerusalem, take on an added dimension, especially since the liberation of Jerusalem during the Six Day War.

Hagai and Zechariah, the two prophets who inspired the building of the Second Temple, lived during the time period known as the 'return to Zion'.  This return of the Exile began after the famous decree of Cyrus (first king of the Persian empire), allowing the Jews to return to Jerusalem (Ezra 1:1-9) after the Babylonian exile.  Although plans to rebuild the Temple in Jerusalem began immediately, those efforts were thwarted by the local population. (Ezra 3:1-4:5)  Only some twenty years later, permission to build was finally granted by Darius the Great, in the second year of his reign.  In that very same year, both Hagai and Zechariah delivered their opening prophecies, encouraging the people that despite their rather pitiful predicament there was hope that this new Temple would one day be greater than the first Temple.  (Hagai 1 and 2, and Zechariah 1, 2 and 3)  During this time period thousands of Jews returned to Zion, (Ezra  2) however many thousands more remained in Babylon. 

In the fourth year of Darius (two years after construction of the Temple began), a delegation of Jews from Babylon came to Jerusalem to inquire regarding the Fast Days. (Zech 7:1-2)

"Shall we continue to weep in the fifth month, (Tisha B'Av) do we abstain ourselves (fast) as we have been doing all these years."
Their question was understandable.  Since the time of the destruction of the First Temple seventy years earlier, the custom in Babylon had been to fast every year on Tisha B'Av.  Now that construction of the new Temple was almost complete it may no longer be necessary to fast.  Considering this, they inquired of the prophet, in order to be informed of the proper laws, whether or not they still needed to fast every year on Tisha B'Av.  For such a simple and logical question, we would expect a straight forward 'yes or no' answer.  Instead, Zechariah answered this question with a complex prophetic answer, spanning two chapters.

"Pay attention to the very same things which the earlier prophets had warned your forefathers when Jerusalem and its surrounding areas were populated and tranquil. (during the good years of the first Temple period)  Execute true judgment, deal loyally and compassionately with one another.  Do not defraud a widow, orphan, stranger, or poor man, and do not plot evil against one another."  Zech 7:7-10

YHVH's answer ---  If and when we fast on Tisha B'Av, we should  remember what happened to Jerusalem, but more importantly, we must remember why the Temple was destroyed. YHVH reminded Israel that the Temple was destroyed for not following the guidance of their prophets.  YHVH is not interested in fasting as a 'religious' act alone but rather that we follow His laws and not repeat the sins of our forefathers.