Rabbi Yonasan Sacks
The character of Tisha B'av manifests itself in two distinct ways. Modeled after Yom Kippur, Tisha B'av is essentially a ta'anis tsibbur (a public fast). The Gemara in Pesachim (54b) explains: "Ein bein Tisha B'av l'yom hakippurim ela shezeh sfeiko assur v'ze sfeiko muttar" - the only difference between Tisha B'av and Yom Kippur is that whereas situations of doubt are forbidden on Yom Kippur, they are permitted on Tisha B'av. Accordingly, the chamisha enuyim, the five afflictions that are forbidden on Yom Kippur are prohibited on Tisha B'av.
Teshuva, essential to Yom Kippur, is a central theme of the krias haTorah on Tisha B'av. "B'tsar l'cha u'mtsaucha kol hadevarim ha'eileh b'acharis hayomim, v'shavta ad Hashem Elokecha v'shomata b'kolo" (Devarim 4) "When you are in distress and all these things have befallen you, at the end of day, you will return unto Hashem your God and hearken to His voice."
Tisha B'av, however, is not merely a ta'anis tsibbur. The gemara in Taanis (30a) teaches, "Kol mitzvos hanohagos b'aveil nohagos b'Tisha B'av". In addition to ta'anis tsibbur, Tisha B'av is characterized as a yom aveilus - a day of mourning. This aspect of the day is emphasized in the haftorah, "asof asifem", "I shall utterly destroy them," which underscores the despair, suffering, and aveilus of K'nesses Yisroel.
The Rambam (Peirush Hamishnayos, Maseches Ta'anis) emphasizes these two aspects of the day. Because Tisha'B'av is a ta'anis tzibbur we refrain from eating and drinking. We sit on the floor and refrain from Torah study to mark the fact that Tisha B'av is a day of mourning.
The Meiri (Yoma 78a) explains that because of the additional element of aveilus, Rabbi Yehoshua Ben Levi refrained on Tisha B'av from wiping his face, hands and feet with a towel - a practice that is permitted on Yom Kippur!
The Minchas Chinuch (mitzvah 312) suggests that the issur rechitzah, the prohibition to wash, for example, links these two elements. Both an avel, as well as someone who observes a taanis tzibbur, are forbidden to wash. However, whereas aveilus prohibits one from washing their entire body, taanis tzibbur forbids one from washing a small part of the body as well.
The Minchas Chinuch further suggests that when Tisha B'av occurs on Shabbos, although the ta'anis is observed on Sunday, various aspects of aveilus apply even on Shabbos.
Emphasizing the nature of Tisha B'av as a day of mourning, the Chassam Sofer (Ohr Hachaim siman 157) maintains that even individuals who must eat on Tisha B'av can be called to the Torah and receive an aliyah. He suggests that the obligation of krias haTorah on Tisha B'Av is not merely a function of ta'anis tsibbur, but reflects the character of the day as a yom aveilus.
The Rambam extends another element of Yom Hakippurim to Tisha B'av. The Gemara (Yoma 81b) explains that one is obligated to begin Yom Hakippurim early, before sunset. The Torah obligation is determined from, "ve'inisem es nafshoseichem betisha bachodesh baerev," indication that Yom HaKippurim is to begin on the ninth of Tishrei towards evening. Most rishonim maintain that this mitzvah is not limited to Yom Hakippurim but applies to Shabbos and yom tov as well. The Rambam does not mention the mitzvah of tosefes Shabbos and yom tov in the Perush Hamishnayos; however, he does emphasize the need to begin Tisha B'av early, "umosifin michol oll hakodesh." According to the Rambam, the mitzvah of tosefes is not a function of kiddush hayom, but rather of taanis tzibbur.
Our commemoration of Tisha Ba'av uniquely binds these aspects of taanis tzibbur and aveilus. May our observance be meorer rachamei shomayim as we await biyas Goel Tzedek bimheira biyameinu.