Tisha B'Av is characterized by Chazal as the "day of tears" (bechiyah le-dorot). The pasuk in Eicha (1:2)- "Bachoh Tibkeh Ba-laylah"- reflects this theme, as its double formulation hints at both destructions of the Beit ha-Mikdash that took place on this tragic day. The Mishnah in Taanit (26b-27a) delineates numerous calamities that befell the Jewish people on this day. Notwithstanding the depth or intensity of the trauma of this day, one might yet perceive this tragic anniversary exclusively within the framework of other established tsomot. In Zekharyah (ch. 8), Tisha B'Av is simply referred to as "tsom ha-chamishi", the fast of the fifth month, just as the 17th of Tamuz is defined as "tsom ha-revii" etc. The Mishnah in Taanit records the fast of the 17th of Tammuz, as well, and establishes that it, too, is a day that commemorates multiple calamities. The Rambam (Hil. Taanit ch. 5), following the Talmudic pattern, groups his treatment of Tisha B'av together with the other established tsomot.
However, there are significant indications that Tisha B'Av is not merely the most severe of the taaniyot, but is qualitatively distinctive, as well. The gemara in Rosh Hashanah (18b) concludes that only Tisha B'Av among the tsomot remains obligatory even in eras of relative prosperity (short of authentic peace and security). The gemara attributes Tisha B'Av's special status to the fact that it marks the anniversary of the double churban. Tosafot (ad. loc s.v. hoiil) questions why the observance of the 17th of Tamuz would not also persist even in less stressful times and conditions in light of the fact that it, too, commemorates multiple losses. Tosafot responds succinctly that the repeated tragedy of the churban justifies Tisha B'Av's incomparable status.
Tisha B'Av's special status is reflected further by the fact that it has emerged as the focal point for the commemoration of other calamities in our history. This is demonstrated clearly by the range of kinot that we include in the Tisha B'Av liturgy. Rashi (Divrei ha-Yamim 2:35:25) notes this phenomenon: "dugma le-Tisha B'Av she-mazkirim kinot al ha-harugim be-yamenu". Moreover, the singular character of Tisha B'Av is underscored by its numerous special halachot, particularly its unusual mix of tsom (Taanit 27a), aveilut (Taanit 13a, 30a; Yevamot 43b; prayer of Nachem etc. ) and a parallel to Yom Kippur (Pesahim 54b).
Perhaps the key to understanding Tisha B'Av is to be found in Eicha's depiction of this day as a "moed" of tragedy (Eikhah 1:15)- "kara alay moed lishbor bachurai". This formulation is responsible for the omission of tachanun on Tisha B'Av, as well as for other halachic nuances and apparent anomalies that differentiates this day from other tsomot. While Aruch Hashulchan and others suggest that this designation refers to the fact that in the future this day, like other tsomot referred to in Zechariah, will be transformed into a true festival (R. H. 18b), it is also possible that it underscores a different motif specifically for Tisha B'Av.
Rambam (Hil. Taanit 1:1-3; 5:1) emphasizes that days of fasting and the commemoration of past calamities are primarily intended as catalysts for teshuvah in one's present circumstances. In his view, focusing on the past is an effective method to highlight the need for greater vigilance and devotion; it is not a commemoration of the past per se. While Rambam himself does not distinguish in this respect between Tisha B'Av and the other tsomot, it is conceivable that Tisha B'Av as a "moed", as the anniversary of the double hurban, demands that we focus additionally on the irreparable loss that has been suffered by the entire nation. Hence, the strong expression of aveilut yeshanah u-derabbim (Yevamot 43b) on this day. A parallel manifestation of mourning on one of the other tsomot would be inconceivable. The recitation of Nachem, which accents this sense of irretrievable loss, is consistent with this theme. As the national day of mourning for the Beit Hamikdash and all that it represented, this day presented an opportunity for all Jews to identify with communities that had suffered persecution and destruction throughout Jewish history. It should be noted that the theme of "mishtatef be-tsaratan shel Yisrael", expressed on this day, is crucial to Jewish identity (Hil. Teshuvah 3:11).
In any case, only Tisha B'Av remains obligatory in the absence of a climate of persecutions, as its past focus continues to be highly relevant. Although the 17th of Tammuz was also a time of multiple sorrows, it was never established as anything but a catalyst for repentance. Possibly, Tosafot intended to convey this idea by contrasting Tisha B'Av's theme of the double churban with the multiple occurrences of the 17th of Tammuz.
Like joyous "moadim" which focus on a particular event or phenomenon in its own right, Tishah B'Av, the "moed" of national mourning, suffering and identification with the historical plight of Kelal Yisrael, is a complex day that is structured to reflect the different facets of its theme. Different stages of aveilut are manifest on this day, and conveyed by various halachot. Like Yom Kippur, it is a day whose complexity provides a foundation for spiritual growth. May we soon witness the transformation of this complex "moed" of loss into a true "moed" of simchah.