The Mitzva to mourn on Tisha B’Av is only temporary, (PLAN B) until it becomes a permanent day of joy and celebration when Mashiach comes as stated in Zacharya 8. Thus, Tisha B’Av is destined to be part of our Halachic tradition, but NOT as a day of eternal mourning.

Therefore, in Megillat Eicha, which is read on Tisha B’Av, Yirmiyahu refers to Tisha B’Av as a MOED – Festival (Eicha 1:15 and in 2:22). That is why we don’t say Tachanun and Selichot on Tisha B’Av (Shulchan Aruch 559).

An example of this idea appears in Yirmiyahu 31 “Then (in the Messianic Era) the young women will dance joyously. Young men and elders will also dance together. I (G-d) will turn their mourning to joy. I (G-d) will comfort them and cheer them from their grief and sorrow.”

This verse clearly echoes the statement expressed by Zacharya. Yirmiyahu is NOT predicting some new, joyous celebration with no roots in the past. Rather, G-d will turn the mourning of centuries of exile into joy and celebration when Mashiach comes.

Why is it that in the time of Mashiach the Fast of the 17th of Tammuz and the Fast of Tisha B’Av commemorating the CHURBAN will become joyous festivals and not just ordinary days? Why is it not sufficient that these fasts will just be cancelled?

The answer is that these tragic mournful days will assume their ORIGINAL intent.

The 17th of Tammuz was the date when G-d gave us the First Tablets of the Ten Commandments. It was supposed to be a day of great joy, celebrating G-d’s gift to us in His own Handwriting!

However, that is the very day that we worshipped the Golden Calf, and Moshe smashed G-d’s Holy Tablets. Moshe had to re-ascend Mt. Sinai and plead for G-d’s forgiveness for the Jewish People. The Second Tablets were not given until Yom Kippur.

When we do TESHUVA and merit the arrival of Mashiach, we will have fully rectified the Sin of the Golden Calf. Thus, the 17th of Tammuz will assume its ORIGINAL character, a day of festive celebration for receiving G-d’s First Tablets.

Tisha B’Av was also supposed to be THE day that the Jews resolved to enter the Holy Land and conquer it. The Sin of the Spies in slandering the Land of Israel, which took place on Tisha B’Av, prevented the Jewish People from entering the Promised Land.

When Mashiach comes, we will have rectified this Sin of the Spies also, and Tisha B’Av will return to PLAN A.

Thus, Tisha B’Av will assume its ORIGINAL intention of intense national joy of celebrating, appreciating, and cherishing the Holy Land of Israel!