One of the most fascinating practices of Tisha B'av is the omission of tahanunim. Typically, these more somber sections of prayer are omitted on festive occasions but are expanded and elaborated upon on fast days. We would expect that on Tisha B'av, the most intense and heart-wrenching fast day of the year, tahanunim would play a prominent role in the liturgy. Instead, they are purposefully left out of the order of prayer.
The commentaries explain that Tisha B'av is unique inasmuch as it is referred to in Scripture as a "moed", a holiday, and is thus entitled to the same exemption from tahanunim that is granted to other festivals. Some might assume that this means that, in the Messianic era, Tisha B'av will attain the status of a moed. The verse cited to substantiate this argument in the Book of Eikha (Lamentations), however, does not support this interpretation:
The enemy established an appointed time (moed) to destroy my young men...
What immediately strikes us about this "proof-text" is the fact that the "holiday" here is one celebrated by the enemies as they crush the Jewish people. It is difficult to see why this tragic phenomenon should serve to establish Tisha B'av as a moed for us. It is clear, though, that Tisha B'av is assigned the title of a "holiday" even now, despite the fact that its tone is far from festive.
It seems, then, that Tisha B'av is indeed a moed, a holiday in its own right. In Jewish terms, a holiday is a time consecrated to reflection on some aspect of our relationship with Hashem. On Pesah, we celebrate God's redemption of the Jewish people from bondage. On Shavuot, we rejoice in the gift of Torah knowledge with which He bestowed us. On Rosh Hashana and Yom Kippur we acknowledge the Kingship, Sovereignty and Mercy of God. On Sukkot we recognize Him as the source of material blessing and security. Generally speaking, this reflection is conducted in an atmosphere of inspiration and joy.
Tisha B'av, however, is a holiday dedicated to reflecting upon the current state of our covenant with Hashem. It is a time set aside for contemplation of the Midat Hadin, the Divine attribute of Justice and its ramifications. Like all moadim, Tisha B'av requires us to deviate from our usual routine and gather together as a community for a transcendent objective. Like all moadim, Tisha B'av is structured around a diminished involvement in workaday activities coupled with an increased involvement in prayer and the study of relevant subject matter (in this case, Eikha, Kinot, etc.). Like all moadim, the liturgy of Tisha B'av is designed to highlight the thematic focus of the day; Tisha B'av has its own Megillah as all Festivals do, extensive kinot are recited in place of Hallel and the lessons of these texts are reinforced with carefully selected Torah and Haftara readings.
Like all moadim, the purpose of Tisha B'av observances is to focus us on specific events in our ancient or recent history so as to lead us toward a greater understanding and appreciation of Hashem's ultimate plan in the world. The events of Tisha B'av, though perpetrated against us by wicked enemies, serve the function of helping us develop a clearer perspective on the stark reality of where we stand before God as a people.
Because the theme of Tisha B'av is an assessment of our covenantal bond with God and the implications of our failure to maintain it, the outcome is a day of mourning and fasting. Were we living in accordance with the Torah and fulfilling our objective as a Kingdom of Priests and a Holy Nation, however, the results of our Tisha B'av reflection would be as positive, exhilarating and uplifting as those of the other moadim of the year.
The Kinnot pick up on and develop this principle in the context of the link between Tisha B'av and Pesah. One of the classic Sephardic Kinot, "Aleikhem Edah Qedosha" contrasts the celebration of Pesah with the mourning of Tisha B'av, in the form of four questions that are presented to the community. The kinah includes the ironic refrain "why is this night different from all other nights?" Another Kinah, recited in Ashkenazic as well as Sephardic congregations, contrasts our experience of Divine Providence as we departed Egypt with our experience of the withdrawal of God's providence as we left Jerusalem as exiles.
Of course, the link between Tisha B'av and Pesah is doubly warranted. First of all, Pesah signifies the beginning of the Jewish nation's relationship with God, and their redemption from the tyranny of human government. This is precisely the opposite of Tisha B'av, which represents a return to pre-Exodus conditions, including subjection to human rule and an inability to perceive God's presence in the world. Second, it is a curious feature of the Jewish calendar that, in a given year, Pesah and Tisha B'av always fall out on the same day of the week, underscoring this parallel even further.
What is most noteworthy, however, is how both Pesah and Tisha B'av are days of reflection upon the fundamentals of God's relationship with His people. In one case, we celebrate the initial covenant that our ancestors in Egypt forged with Hashem and the miraculous transformation and redemption that resulted therefrom; in the other, we consider our abandonment of the selfsame covenant and mourn the current unredeemed state of our nation in exile. Surface-level differences in observance and atmosphere notwithstanding, the respective themes of Pesah and Tisha B'av are ultimately two sides of the proverbial coin.
In summary, Tisha B'av is, indeed, a Moed, in the sense that it is a period of time consecrated to reflection on our relationship with God and His Providence. In particular, Tisha B'av deals with our national covenant with Hashem and the principles of Divine Justice associated with it. No Tahanun is recited on Tisha B'av because it possesses the essential quality of a holiday, despite the fact that, in our current state, the tone of Tisha B'av is mournful and depressing.