Yesh shom yomim shekol Yisroel misanim bohem mipnei hatzaros she'iraru bohen k'day le'orer halevavos uliftoach darkei hateshuva...
There are days when the entire Jewish people fast because of the calamities that occurred on them in order to arouse the hearts and open avenues of repentance
-Rambam, Hilchos Ta'anios 5:1
The Rambam defines very clearly the purpose of the historical fast days, amongst which Tisha B'av is the most prominent: to arouse the hearts and open avenues of teshuva. At first glance, one could adequately paraphrase the Rambam by saying that the ta'anis should be devoted to teshuva. Upon closer examination of the phrase "darkei teshuva - avenues of repentance", however, it becomes clear that the Rambam's expectations for us on a ta'anis are much, much higher.
The same phrase "darkei teshuva" appears in the Rambam's Hilchos Teshuva:
Midarkei hateshiva li'hiyos hashov tzoek tamid lifnei Hashem b'vechi v'tachanunim v'oseh tzeddaka k'fi kocho u'misracheik harbeh min hadavar she'chata bo umeshaneh shemo, klomar she'ani acher v'aini oso ha'ish she'asah osan ha'ma'asim u'meshaneh ma'asav kulan letova u'l'derech ye'shara v'goleh mi'me'komo
The avenues of repentance include that the penitent constantly cries out before Hashem with tears and pleading, and engages in charitable works to the best of his ability, distances himself very much from his sin, changes his name, thereby stating that I am different, I am not the person who performed these actions, he improves all his actions and exiles himself from home...
- Rambam Hilchos Teshuva 2:4
It is abundantly clear based on this passage in Hilchos Teshuva that darkei teshuva for the Rambam signifies a permanent lifestyle of teshuva. This observation affords us a fuller and deeper understanding of the Rambam's charge for ta'anios. Indeed, a fast day should be devoted to teshuva. But that is only the beginning. The ta'anis should open darkei teshuva, it should introduce enduring change in our lives. The twenty four hours of introspection are not merely a time for teshuva; as a result of these hours we should embark upon a new lifestyle of teshuva. Thus ta'anios in general and Tisha B'av in particular are a time to identify actual changes that we can and b'siyata d'Shmaya will implement in our lives.