After Yetzias Metzraim, Bnei Yisroel sent spies to Eretz Yisroel to see if the land was good. The spies came back and said lashon hara about Eretz Yisroel. Bnei Yisroel cried and said they would rather go back to Mitzrayim.
Hashem was upset and told Bnei Yisroel that the day they cried for no reason, Tisha B'Av, would be a day of crying and mourning for Bnei Yisroel forever.
Because Bnei Yisroel cried for no reason, Hashem declared that their generation would not enter Eretz Yisroel. Only their kids were allowed to enter.
The first and second Beis Hamikdash were destoyed on Tisha B'Av.
Many other terrible tragedies befell Bnei Yisroel on Tisha B'Av throughout the years.
There are a few things we cannot do on Tisha B'Av:1) Eat or drink 2) Bathing or washing 3) Use cream or lotion 4) Wear leather shoes
We eat a Seudah Hamafsekes before Tisha B'Av, just like by Yom Kippur.