The 9th of Av is considered the saddest day in the Jewish Calendar, and is traditionally marked by a 24-hour fast. Nothing is to be consumed from nightfall to nightfall, and the day is supposed to be one of mourning and prayers.
On this day we commemorate the destruction of both the first and second Temples in Jerusalem in 587 BCE and 70 CE respectively.
For some, this might seem like overkill, as bemoaning an edifice lost long ago seems like something we should have gotten over by now.
But in Judaism things have meaning, and remembering and commemorating the Temples which were the focal point of the entire religion and people for more than 800 years is something to remember indeed. The point is not just to remember, but to live in the spirit that those two Temples represented throughout Jewish history: spirituality, a united people, and a proud legacy.
Plus, fasting is so “in” these days—at least, intermittent fasting—and if you have your history pushing you in that direction too, all the better.