Tisha B'Av or Tish'ah b'Av (Hebrew: תשעה באב or ט׳ באב, tish‘āh bə-āv) is the saddest day on the Jewish calendar, is it a month and a day fraught with tragedy.

Tisha b'Av is a day of national mourning. It is a day of fasting and rituals of mourning are observed.

A multitude of national calamities occurred on Tisha b'Av throughout the history of the Jewish People.
During the time of Mosess, as the refugee Israelites traveled the Sinai Desert, it was on the 9th of Av that they accepted the slanderous report of the 10 Spies, and the decree was issued forbidding them from entering the Land of Israel. (1312 BCE)
 
Both the First Temple and the Second holy Temple in Jerusalem were destroyed on the ninth of Av. The First Temple at the hands of the Babylonians, led by Nebuchadnezzar. 100,000 Jews were slaughtered and millions more exiled. (586 BCE)
 
The Second Temple was destroyed by the Titus led Romans. Over two million Jews perished, and another million were exiled. (70 CE)

And as the prophet Yirmiyahu foretold, Jerusalem was ploughed over - on the ninth of Av. The Temple area and its surroundings were stamped on and squashed by the Roman general Turnus Rufus. Jerusalem was rebuilt as a pagan city -- renamed Aelia Capitolina -- and access to the Jews was forbidden.
 
It was on t he 9th of Av that the Bar Kochba revolt was crushed by Roman Emperor Hadrian. The city of Betar -- the Jews' last stand against the Romans -- was captured and liquidated. Over 100,000 Jews were slaughtered. (135 CE)
 
On the 9th of Av 1492, the Spaniards expelled the Jews from their country, a culmination of the Spanish Inquisition that brought the death of countless Jews.
 
On the 9th of Av of 1914 - a World War began. It was on the eve of Tisha b'Av in 1914 that Germany declared war on Russia, a war that brought on economic calamity for Germany, and engendered an anger that festered, and a resentment that set the stage for the Holocaust and the murder of six million Jews.
 
The 9th of Av 1942 recorded the first killings started at Treblinka: "The first transport of 'deportees' left Malkinia on July 23, 1942, in the morning hours. It was loaded with Jews from the Warsaw ghetto. The train was made up of sixty closed cars, crowded with people. The car doors were locked from the outside, and the air apertures barred with barbed wire."
The 9th of Av is indeed a sad day in the history of the Jews.  It is also promised to be the day of redemption, when bitter tears will be replaced by song and dance and lightness of the heart.  It is believed by many that the Jewish Messiah will first show himself on the 9th day of Av.