Saturday evening begins the ninth day of the fifth month in the Jewish calendar, which is “Tisha B’Av, the Fast of the Ninth of Av, is a day of mourning to commemorate the many tragedies that have befallen the Jewish people, many of which have occurred on the ninth of Av,” according to Judaism 101.
“Tisha B’Av primarily commemorates the destruction of the first and second Temples, both of which were destroyed on the ninth of Av (the first by the Babylonians in 586 B.C.E.; the second by the Romans in 70 C.E.).
“Although this holiday is primarily meant to commemorate the destruction of the Temple, it is appropriate to consider on this day the many other tragedies of the Jewish people, many of which occurred on this day, most notably the expulsion of the Jews from Spain in 1492 and from England in 1290.” —Judaism 101
There is another tradgedy that concerns the ninth of Av. “The year is 1313 BCE. The Israelites are in the desert, recently having experienced the miraculous Exodus, and are now poised to enter the Promised Land. But first they dispatch a reconnaissance mission to assist in formulating a prudent battle strategy. The spies return on the eighth day of Avand report that the land is unconquerable. That night, the 9th of Av, the people cry. They insist that they’d rather go back to Egypt than be slaughtered by the Canaanites. G‑d is highly displeased by this public demonstration of distrust in His power, and consequently that generation of Israelites never enters the Holy Land. Only their children have that privilege, after wandering in the desert for another 38 years. — Chabad.org
Those things happened a long time ago. More recently,”World War II and the Holocaust, historians conclude, was actually the long drawn-out conclusion of World War I that began in 1914. And yes, amazingly enough, Germany declared war on Russia, effectively catapulting the First World War into motion, on the 9th of Av, Tisha b’Av.” — Chabad.org
So, how long must we remember things of the past? As long as it takes that they are not repeated.
Chabad.org put it this way: “What do you make of all this? Jews see this as another confirmation of the deeply held conviction that history isn’t haphazard; events—even terrible ones—are part of a Divine plan and have spiritual meaning. The message of time is that everything has a rational purpose, even though we don’t understand it.”