Tisha B'Av is often referred to as Israel's 9/11 because of its calendar scheme, but more importantly because of how tragic and destructive events on this day deeply changed the political, religious, economic, and cultural trends of the country. 

Starting with being denied entry into the Promised Land for their lack of faith, desire to silence G3D's Messengers, and plan to fight mortal enemies on their own, this trend was also seen in the destruction of both the First and Second Temples of Jerusalem. 

Israel's Day of Mourning is set aside to remember the events that led to G3D's Presence and Power leaving His People, and the devastation that fell on that Prophetic Date.