The sin of the spies took place on Tisha B'Av when the entire nation cried all night after hearing the evil report concerning Eretz Yisrael. Hashem said that because they cried for nothing, בכיה של חינם, this day of Tisha B'Av will be a day of crying for generations, בכיה לדורות.
One such example of a tragic event on this date was the expulsion from Spain in 1492. There was a Jewish presence in Spain for over 1400 years from the time of the destruction of the second Temple. Most are familiar with the term, " the golden age of Spain." However, of the 1400 years the Jews lived in Spain, only 200 years of their stay was really pleasant.
This occurred when the Muslims had overthrown the Christian influence. They were kind to the Jews who helped the Muslims establish their rule. When the Christians again regained power, they were extremely oppressive to the Jews. Those who did not actually convert to Christianity, had to pretend to be Christian, while practicing Judaism in secret.
When the situation became unbearable, 300,000 Jews were expelled on Tisha B'Av, 1492.
Perhaps one of the lessons of this period of the "nine days" is that wherever Jews lived outside of Israel, it always ended in assimilation, expulsion, or extermination. We must never stop giving thanks that we have returned home to Israel.