On Tish'a b'Av we mourn for the destruction of the ﬁrst and second Batei Mikdash. The Talmud in Taanit
(30) states that all Mitzvot that apply to a mourner during the Shiva also apply to the entire Jewish people on Tisha b'Av.
In other words, according to Halacha we are all sitting Shiva on Tisha b'Av. But, how can this be? For the Halacha states that if a close relative has died and we did not know about it until after 30 days, there will be no formal Shiva. We would sit Shiva symbolically for only an hour, and then get up and go about our regular routine. How then, can we sit Shiva on Tisha b‘Av for tragic events that occurred thousands of years ago?
Rav Soloveitchik answers this question with a statement in Talmud Yerushalmi which states, "Every generation in which the Beit HaMikdash has not been rebuilt is like the generation in which it was destroyed." Thus, we are not mourning on Tisha b'Av only for tragic events that happened thousands of years ago, but we are also mourning for the ongoing tragedy of not having the Temple rebuilt in our time.
Why is the Beit HaMikdash so important to us as Jews’? The Chofetz Chayim explains that more than half of the 613 Mitzvot depend solely on the Mikdash. This means that for close to 2000 years we are unable to fulﬁll the majority of Mitzvot. Therefore, we need the Beit HaMikdash in order to become spiritually healed and whole again.
The Mitzva to mourn on Tisha b'Av is only temporary, until it becomes a permanent day of joy in the Messianic Era as stated in Zacharia 8:19. Thus, Tisha b'Av is destined to be part of our Halachic tradition, but not as a day of eternal mourning. In Megilat Eicha, which is read on Tisha b'Av, Yirmiyahu (Eicha 1:15 and in 2:22) refers to Tisha b'Av as "Moed" (a Festival). That is why we don't say Tachanun and Selichot on Tisha b'Av (Shulchan Aruch 559:5).
What are the signs of the approaching Messianic Era? And how can we hasten Moshiach’s coming? The Talmud states that the Jews returning from exile and the turning of the land of Israel green is the key sign for the beginning of the Redemption. "There is no clearer sign then when the Land of Israel gives its produce abundantly, then the end of the exile is near, (Sanhedrin 98b, Rashi).
The Vilna Gaon told his disciples that Moshiach will come after the majority of the Jews in the Diaspora come to Israel. Thus, every Jew who makes Aliyah hastens the Moshiach’s coming, thereby transforming the Fast of Tisha b'Av into a Feast.
For close to 2000 years our land rejected all would-be conquerors and remained desolate and barren. The Sifra explains that the Torah's curse of the land during our long exile, "I will make the land desolate" (Parshat Bechukotai), is actually a blessing in disguise. Because we didn't have to worry when we went into exile that our enemies would settle our land. Therefore, the greening of Israel is a clear signal that G-d's decree of "desolation" for the land is over and the Redemption is near. The Land of Israel had to go into "hibernation," waiting for us, her children, to return from exile.
The prophet Yirmiyahu saw the coming Redemption of the Messianic Era and its celebration as being an even greater event than the Exodus from Egypt. As the Prophet says in Yirmiyahu l6:l4,l5 - "Days are coming, says Hashem, when it will no longer be said, ‘As Hashem lives Who took out Israel from Egypt’, but rather ‘As Hashem lives, Who took out Israel from all the lands where Hashem dispersed them.’ And Hashem will return them to their own land, which I gave to their forefathers."
According to HaRav Mordechai Eliyahu zt”l: and the Lubavitcher Rebbe zt”l (and many others), we are presently in the dawn of the Messianic Era. If Moshiach should arrive before Tisha b'Av, then we will be feasting and celebrating on that day, instead of mourning and fasting.