The Tur (Orach Chaim 549) introduces the halachos of fasting and fast-days by describing the four fasts of 17 Tammuz, Tish'a B'Av, Tzom Gedalya and 'Asara B'Teiveit as days of mourning commemorating tragic events in Jewish history. Whereas the Kinot and Shiv'a-like mourning practices on Tish'a B'Av underscore this theme on this saddest day of the year, the Tur teaches that the purpose of the other fast days as well is to mourn the various stages of the destruction of the Temple, the removal of Jewish sovereignty over the totalilty of Eretz Yisrael and the dispersion of the Jewish nation. Rav Yosef Dov Halevi Soloveitchik zt"l maintained that in light of this aspect of these days, practices of the "12 months" period of mourning for a parent apply on all of them. [See Peri Megadim (quoted by Bei'ur Halacha 551:s.v. meiRosh Chodesh) who opines that even the practices of the "Nine Days" apply on these days.]
Rambam (Hilchot Ta'aniyot Chapter 1) -- as noted by Chasam Sofer (Orach Chaim 208) and later by Rav Soloveitchik -- highlights a different aspect of these days -- that of repentance. By contemplating the past tragedies and realizing that any generation that has not merited the restoration of the Mikdash is also considered guilty of the same crimes which led to its destruction, the individual is spurred to repent for the misdeeds of both his ancestors' and his own generations.
Clearly, both aspects, mourning and repentance, are present in varying proportions on all the fast-days. The Selichot recited on the other fast-days and the Kinot of Tish'a B'Av recall the various tragedies which occurred on that particular day. The Torah reading of Va'Yechal and the Haftorah of Dirshu Hashem B'himatz'o read on all of these fast-days call to repentance. The repetition of the 13 Middot HaRachamim throughout the Selichot on the other fast-days serve as an intense prayer for forgiveness. It would appear that on the evening and morning of Tish'a B'av the mourning aspect dominates: the Torah reading and Haftorah of Shacharis warn of the dire consequences of abandoning the D'var Hashem. The Kinot do not include the 13 Middot HaRachamim. Only at Mincha do the Keriat HaTorah and the Haftorah switch to those of a regular fastday. This seems to be consistent with the theme found in pos'kim that some of the restrictions of mourning are relaxed after noon of Tisha B'Av as we express our confidence that through our repentance we are assured to see the fulfillment of the prophecies of consolation: total return to Tzion and rebuilding of the Mikdash. [See Shiurei HaRav 'al 'Inyanei 'Aveilut v'Tish'a B'Av compiled by Rav E. Koenigsberg, especially Inyanei Tish'a B'Av (Simanim 1 & 20) for further analysis of these themes.]
The fact that the first half of the day does not focus on repentance but exclusively on mourning may perhaps be explained with an analogy from the t'shuva process itself. As is well known, repentance as formulated by Rambam consists of three fundamental components: 1) regret, 2) confession and 3) commitment not to return to sinning. Sincere regret requires intense contemplation of the sin: both concerning the Infinite power and exaltedness of the Legislator of human conduct and the act of rebellion against His wishes. The singularly-focused mourning over the past and present destruction serves as an intense impetus for sincere regret over the sins causing it. Contemplating, reciting and seeing before the mind's eye the devastating physical effects of the sin highlight the parallel, enormous, spiritual distance formed between Creator and created by these actions. Only after this realization can the mourner truly turn toward repentance. This theme is expressed by the Slonimer Rebbe zt"l in his Nesivos Shalom, interpreting a passage in T'hillim (139:8): "If I soar to the heavens, there You are; and if a descend to the Pit, behold, You are there [too]." A sinner, even while wallowing in the depths of his crimes, realizes that this same emotion can serve as the greatest of incentives to soar back heavenward, realizing that Hashem is always with him and awaits his return.
The Haftorah for this Shabbat, whose first word, "chazon", has traditionally been incorporated into the name of the Shabbat on which it is read (Shabbat Chazon), also stresses both of these aspects. Yeshaya first prophetically mourns the future destruction:
"Why should you be further smitten when you increase your straying; all heads are ill; all hearts are in pain. ... Your land is desolate, your cities are burned in fire, right before you, strangers cosumed your land, and it is desolate being overturned by strangers. And the Daughter of Tzion shall remain as a booth in the vineyard ... as a city besieged.is your land. If not that Hashem [Lord of] Hosts left a remnant, we would have been as S'dom, similar to 'Amora! (1:5-10)"
This is quickly followed by Yeshaya's fervent call for repentance:
"Cleanse yourselves, purify yourselves, remove the evil of your ways from before My eyes, cease harming. Learn to improve yourselves, seek out justice, support the victim, judge the orphan, advocate for the widow. Let us investigate, says Hashem, if your sins will be as scarlet, as snow shall they be whitened; if they will be reddened as crimson, as wool shall they be. If you would but will [to change] and hearken, the good of the land shall you eat! (1:15-19)"
Perhaps the fact that we traditionally read this Haftorah before Tish'a B'Av itself serves to highlight that the entire extended mourning period surrounding the fast-day, the week of Tish'a B'Av according to the Talmud and, by common custom, including the Three Week period from 17 Tammuz, also is an appropriate time for both themes: mourning and repentance. The Kozhnitzer Maggid zt"l quoting Rav Elimelech of Lizhensk zt"l alludes to this duality by commenting on the verse from Eicha, after which this mournful period is named: "Kol rod'feha hisiguha bein ha'metzarim" -- "all of her [Tzion's] pursuers overcame her between the straits." The Maggid interprets this verse homiletically as "all who pursue Hashem [rod'fei kah] during this time period of Bein HaMetzarim will succeed in achieving their spiritual quest."
In the dual merit of our mourning over the physical and spiritual devastation brought about through our past and present misdeeds and hopefully the thoughts and actions of repentance motivated by this mourning, may we see the complete "simchas Y'rushalayim" in our days!