On Friday we celebrate Tu B'Av. Some explain that the joy of Shabbos Nachamu is really rooted in the joy of Tu B'Av since when the ninth of Av occurs on Sunday, Shabbos Nachamu and Tu B'Av coincide. Even when they do not, their proximity warrants a festive mood similar to Tu B'Av.
In the Talmud (Ta'anis 26b), R. Shimon ben Gamliel famously declares: "there were no holidays in Israel as great as Tu B'Av and Yom HaKippurim." The Gemara (ibid. 30b-31a) states that Yom Kippur understandably is a holiday as it is the day of forgiveness and atonement, but why is Tu b'Av such a happy occasion? The Talmud gives six reasons:
Several of these events are really just slight respites from tragedies of massive proportions. Are these really cause to celebrate? Furthermore, is there a common theme linking all six events?
In answer to these questions, it would appear that Tu B'Av, coming as it does so soon after Tish'a B'Av and with its above-mentioned link to Shabbos Nachamu, is the "headquarters" of that which will undo the damage which brought about the exile. This perhaps is the root cause of the celebration.
The process of teshuva, which is a crucial component for quickening the redemption, requires undoing the causes of the exile. In addition, once the teshuva process has begun, and, to a large extent, even before, Divine mercy advances the redemption and reverses its evils. As Yeshayahu states: "For a short moment I left you, and with great mercy I will gather you" (Isaiah 54:7). Each one of the events we commemorate and celebrate on Tu B'Av falls into one of these two categories, repentance and Divine mercy.
The first two events demonstrate Divine rachamim even in times of Divine wrath. Part of the generation which rejected the gift of Eretz Yisrael was spared. The bodies of the many Jewish casualties of the utter Roman destruction of Beitar did not decay and were eventually allowed to be buried. Even within the period of enormous sorrow, Hashem demonstrated His love for us and indicated that even if we are undeserving, He will still advance our redemption - "and He brings a redeemer to their descendants for the sake of His name, with love."
The next three events demonstrate the Jewish people's movement toward undoing the causes of destruction and the exile. The destruction of the first Temple was caused primarily by violations of the three cardinal sins, chief in the list being avoda zara (Yoma 9b). Part of this campaign of widespread avoda zara was Yorov'am's banning of aliya to Jerusalem to worship Hashem there. Hoshei'a's removal of his roadblocks represented an opportunity to change.[1] The destruction of the second Temple was caused primarily by baseless hatred tearing the unity of the Jewish people apart (ibid.). Regarding the various decrees forbidding marriage of one tribe and another - ordained in order to preserve the integrity of the inheritance of each tribe or to punish the reprehensible actions of one of the tribes - even though each was made for an important reason, they still were detrimental to the unified fabric of Klal Yisrael. Their repeal represented a step toward achieving that elusive unity. Of course, these events occurred long before the destruction of the Batei Mikdash and accompanying exiles. But perhaps these events injected into the fabric of B'nei Yisrael an extra boost toward the ability to be unified.[2]
How does the celebration of the siyum k'risas ha'eitzim fit into the pattern? The Torah warns us that one of the causes of exile is doing mitzvos by rote, as a burden or chore to be dispensed with quickly, moving on to our "real" pursuits. The tochacha in parashas Ki Savo threatens exile as a consequence of "your not serving Hashem, your G-d, with joy and happiness of heart" (28:47). Megillas Eicha (1:3) states "Judah was exiled out of pain and great servitude". Among the other interpretations of this verse, some suggest that Yirmiyahu is indicating one of the reasons for the exile. We served G-d, not out of joy with a sense of gratitude for the immense opportunity, but with an attitude of viewing that service as painful and even a burden. The immense joy accompanying the siyum hamitzva of cutting the wood for the altar - which actually serves as one of the sources for the concept of celebration at a siyum maseches (see Yam shel Shlomo, Bava Kamma 7:37) - is the exact opposite of that attitude. Mitzvos are our mission in life to be relished, performed with zest and happiness, and celebrated upon their completion. The next step is to immediately seek other mitzva opportunities. This is noted as well by the Gemara as it ends its discussion of this celebration by emphasizing the need to increase one's Torah learning at that precise time.
Several of these themes are found in parashas VaEschanan, the parasha always read on Shabbos Nachamu, as well. Moshe's living example of passion for mitzvos is manifest throughout the parasha. His pleading with Hashem to enter the holy land was, as the Gemara in Sota (14a) teaches us, not to enjoy of its beautiful physical bounty but to pursue its unique mitzvos not relevant outside of Eretz Yisroel. Moshe's seizing the opportunity to designate the three arei miklat in the East bank of the Jordan river (4:41 ff.) - even though he knew they would not be effective until the days of his student, Yehoshua - also serves as an appropriate model for us to run after mitzvos even if we are not able to complete them. The portion "ki tolid banim", read on Tisha B'Av, adjures us to use the opportunities that the exile presents us to return to G-d and wholeheartedly serve Him (see 4:29 ff., see also 6:5).
May our return to Hakadosh Baruch Hu inspired by the tragedies of the exile and renewed enthusiasm for Divine service as well as a realization of G-d's infinite mercy serve as merits to rush the final redemption!
[1] Commentaries note the Gemara in Gittin (88a) that his actions actually indirectly caused the exile of the Ten Tribes, since the Jews in the kingdom of Yisrael did not take advantage of this opportunity. Why then is this cause to celebrate? Perhaps the simplest answer is that we celebrate the opportunity, which itself is significant.
[2] See Gemara Sanhedrin (19b) concerning Yosef, Boaz, and Palti ben Laish for an example of this concept.