The three weeks from Shiv'a Asar B'Tamuz until Tisha B'av mark a unique period of universal mourning for Klal Yisroel. Our anguish is most intense on Tisha B'av, the day of the destruction of the second Bais HaMikdash. Yet, even on Tisha B'av itself, we experience various degrees of aveilus. During the beginning of the day, our aveilus is most profound and intense. At that time, in addition to the general restrictions which apply throughout the day, we do not wear tallis and tefillin, we refrain from sitting on a bench or chair, and we omit "tiskabel tzeloshon" in kaddish. After chatzos, however, the intensity of our aveilus diminishes. At that point we wear tallis and tefillin, sit on chairs and recite the entire kaddish.
From a historical perspective, our observance of Tisha B'av seems paradoxical. The Beis HaMikash was set afire toward the end of Tisha B'av and continued to burn on the following day. In fact, the Talmud Yerushalmi (Taanis 4:6)records that Rabi Abin fasted both on the ninth as well as the tenth of Av. Furthermore, Tosafos suggests (Megilla 5b) that Rav Yehuda HaNassi maintained that the destruction of the Beis HaMikdosh should be commemorated on the tenth of Av, rather than on the ninth. It would therefore seem more appropriate to express our most profound aveilus at that time. Yet, precisely at the moment that the Bais HaMikdash was burning, the intensity of our aveilus lessens.
Rav Soloveitchik zt"l explained that our Tisha B'av observance can be understood in light of the following midrash: Perek 79 of Sefer Tehillim begins, "Mizmor leasaf. Elokim, bau goyim benachalasecha, timu et heichal kodshecha," -a mizmor describing how the non-Jewish nations defiled our Beis HaMikdosh. The Middrash asks, would it not have been more fitting to use the term "kinah", a lamentation, rather than "mizmor"? In what sense is the description of the Beis HaMikdosh praising Hashem? The Midrash answers, "kilah Hakadosh Baruch Hu chamaso b'eitzim va'avanim she-b'beiso" (see Rashi and Tosafos to Kiddushin 31b) HaKadosh Baruch Hu chose to vent his wrath at the sticks and stones of the Beis HaMikdosh and thus Bnei Yisroel survived. As tragic as the vents of the churban appear, we can find comfort in the fact that HaKadosh Baruch Hu's anger was not directed at Klal Yisroel itself.
Rav Soloveitchik zt"l explained that in this sense we can now understand out Tisha B'av observance. The most precarious and dreaded moment of the churban was prior to the actual destruction of the Beis HaMikdosh. The anger of Hashem was apparent , and Klal Yisroel itself feared imminent destruction. Only when the Beis HaMikdosh was set afire did Bnei Yisroel fully realize that they would be spared. In this sense the destruction of the Beis HaMikdosh was an act of chessed of Hakadosh Baruch Hu; one which is marked with mizmor (praise). Accordingly, our aveilus on Tisha B'av is most intense for the beginning of the day, and yes, as the Beis HaMikdosh continued to burn our aveilus diminishes.