“B’simkha Yegilun Kol Hayom” (In Your Name they will rejoice all the day), is an acronym for the word BiKHiYa –weeping. (Likutei Moharan I: 175)
The Three Weeks, The Nine Days, and especially Tisha B’Av are hard times for all of us. It is a time of hard judgments and retribution. On Tisha B’Av we mourn over both the First and Second Temples. Yet, despite this, we do not recite Tachanun on this day. It is called a festival, as in, “They declared me a festival” (Eikha 1:15).
Hidden within its very essence, Tisha B’Av is mamash a day of hope. The fact that we mourn testifies to our belief that we will one day be redeemed. We all know that crying over the past is useless. Yet, on Tisha B’Av, our Sages tell us to cry. So these tears cannot be out of despair. They must be out of a hope that one day our deliverance will come.
In the very center of our pain, there really is a great hope. It is correct to call Tisha B’Av a festival, as it says, “Hashem-Tzva’ot says, ‘The fast days of the fourth, fifth, seventh and tenth months are to become times of joy, gladness and cheer for the house of Yehudah” (Zechariah 8:19). We do not recite Tachanun on Tisha B’Av because deep within the sadness is the hope of redemption.
Rabbi Mordechai of Slonim writes:
When Pharaoh’s daughter found baby Moshe in the Nile, the verse relates: ‘And they saw the child, and behold, it was a crying boy. And they said, this is one of the Hebrew children’ (Exodus 2:6). The verse seems to imply that from the very manner of his cries they could tell that he was a Hebrew. This is because his cries had a hopeful ring to them. They heard his cries, and realized that this was a Jew.” (Mayanot HaNetzach)
Base on In All Your Ways, Collected Discourses of Rabbi Yaakov Meir Schechter