The ninth of Av is the day that the other side has the most power to cause negative events. There are many sources online where we can get more information about this significance day.
Many negative events occurred on this day but the most significant is the destruction of both Holy Temples and the exile of the Shechina from her dwelling place, the Holy Temple. Without the Shechina in the Holy Temple we are like children without a mother to support and comfort us. On that day, we fast from sundown to sundown, sit on the floor like mourners and disconnect from all physical and even mental pleasure. We do not study Torah or Zohar because it gives us pleasure. We should feel the pain of the absence of Light from our lives.
The only study we can do should be related this day and the study of Zohar Eicha (not available in the English Zohar).
You can find a copy here: http://dailyzohar.com/zohar-eicha-%D7%96%D7%95%D7%94%D7%A8-%D7%90%D7%99%D7%9B%D7%94/
You can scan and listen to the Zohar above but the study will be published tomorrow.
While the negative side is most powerful on ninth of Av, this day is also the birthday of Mashiach. A great Light comes out of darkness. Because of this aspect of the day we rise up of the floor in the middle of the day and stop the process of mourning of the destruction of the Holy Temple. We do continue the fasting until the end of the day but now it’s because we want to have a clean vessel to receive the great Light of Mashiach and the Third Holy Temple.
More on this subject later today.