On the Ninth of Av (Tisha B'Av) it was decreed on our ancestors that they would not enter the Land (of Israel), the Temple was destroyed the first and second time, Beitar [the stronghold of the Bar Kochba rebellion] was captured, and the city (of Jerusalem) was plowed under.
Talmud Taanis 4
As the anniversary of the most tragic events in Jewish history, the ninth of Av is the most important day of mourning in the Jewish year. Like all fast days, the basic purpose of Tisha B'Av is for us to meditate upon the tragedies that have come about through our sins and to commit to rectifying those errors. As Maimonides writes (Hil. Taanios 5:1):
There are days when all Israel fasts because of the troubles that occurred on them, in order to arouse the hearts to begin the ways of repentance and to be a reminder of our evil deeds and the deeds of our ancestors that were like our deeds today which brought them and us these troubles. For by remembering these things we are brought to return to the good, as it says (Leviticus 26:40), "And they shall confess their sin and the sin of their ancestors."
The sources enumerate a number of different sins that were the root causes of the destruction, and which should be the primary focus of repentance on these days. These include:
- Unjustified hatred of our fellow Jews (sinas chinam). We must work to feel love towards our fellow Jews, and also for all human beings.
- Murder. We should work on ourselves to respect our fellow man and see in him the image of God that exists in every person. Even publicly shaming another person is likened to murder.
- Idolatry. We must recognize that only God is the cause of good and bad. No person or thing can hurt you or help you unless it is the will of God. Nothing else has any real power in the world. We should focus on developing our relationship with HaShem and to realize that He cares about each and every one of us and hears our prayers.
- Immorality. Not only must we avoid outright acts of immorality, but we must also train ourselves to avoid circumstances and situations that can lead us in that direction.
- Neglect of Torah study. We must recognize that the Torah is God's direct revelation to us. As such, it is fundamentally different from all other forms of knowledge. From the Torah we learn what our purpose is in this world and how to achieve that purpose. Our attitude towards the Torah must express this recognition. We must not treat the study of Torah like any other form of study.
Note: Part of this post is a repeat of material from a previous post: Asara b'Teves - Mourning the Loss of the Temple