What is the significance of the the 9th of AV / Tisha b’Av ( the Jewish day of mourning ) to the nation of Israel?

 The Spies Returned With a Bad Report

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 Av and 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.

Both Jewish Temples were destroyed

The First Temple was also destroyed on the 9th of Av (423 BCE). Five centuries later (in 69 CE), as the Romans drew closer to the Second Temple, ready to torch it, the Jews were shocked to realize that their Second Temple was destroyed the same day as the first.

The Battle at Betar Was Lost

When the Jews rebelled against Roman rule, they believed that their leader, Simon bar Kochba, would fulfill their messianic longings. But their hopes were cruelly dashed in 133 CE as the Jewish rebels were brutally butchered in the final battle at Betar. The date of the massacre? Of course—the 9th of Av!

The Romans Plowed the Beit Hamikdash

One year after their conquest of Betar, the Romans plowed over the Temple Mount, our nation’s holiest site.

The Jews Were Expelled From England

The Jews were expelled from England in 1290 CE.

The Jews Were Banished From Spain

In 1492, the Golden Age of Spain came to a close when Queen Isabella and her husband Ferdinand ordered that the Jews be banished from the land. The edict of expulsion was signed on March 31, 1492, and the Jews were given exactly four months to put their affairs in order and leave the country. The Hebrew date on which no Jew was allowed any longer to remain in the land where he had enjoyed welcome and prosperity?

World War I Unleashed Destruction

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.