WHY DO WE FAST ON 9TH OF AV?

Unless Moshiach (the Messiah) comes first, the fast of 9th of Av will start next week on Monday afternoon and will last for about 25 hours until after nightfall on Tuesday.

The fast of 9th of Av commemorates 5 tragedies that befell the Jewish people on that date:

It was decreed that the generation which left Egypt would remain in the desert for 40 years and not enter the land of Israel, after believing the inaccurate report of 10 of the 12 spies over 3,000 years ago.

The first Bet Hamikdash (Holy Temple) was destroyed on 9th of Av almost 2,500 years ago.

The second Bet Hamikdash (Holy Temple) was destroyed on 9th of Av about 1950 years ago.

The city of Betar was captured and tens of thousands of Jews were killed about 1,800 years ago.

The wicked Turnus Rufus plowed the site of the Bet Hamikdash  (Holy Temple) and its surroundings and renamed it Aelia Capitolina, about 1,800 years ago.

Since these tragedies occurred on 9th of Av, it was decreed as a day of fasting and mourning.

Source: Kitzur Shulchan Aruch 121:5

Other tragedies that happened on 9 B'Av:

4,000 Jews were expelled from England by King Edward I in the year 5050 (18 July 1290).

300,000 Jews were expelled from Spain by Isabella of Castile and Ferdinand II of Aragon in the year 5252 (2 August 1492).

Word War 1 started in 5674 - 1 August 1914 - with Germany declaring war on Russia.