The Tragedy of the First of Av
A religious Jewish group went to one version of Aharon Hakohen's burial spot on his Yahrtzeit undercover in Jordan, according to Ladaat.net, where more pictures are shown. Others dispute this being his burial spot.
The first of Av is Aharon Hakohen's Yahrtzeit. After his death, the king of Arad fought against the Jewish people. What was the sin that caused this?
The Ya'arot Devash explains that the Jewish people sinned by not praying for Aharon Hakohen not to die before he died and only waiting until after he died to cry about his death.
The Yerushalmi quotes an opinion that says that the first Beit Hamikdash was really destroyed on the first of Av. However, the Ya'arot Devash asks from Esther Rabba 7:11 how Tamuz and Av could not accept any more major tragedies and couldn't accept Haman's lottery to fall upon them. Similarly, we should say that the first of Av had this terrible tragedy of Aharon's death and it should not accept another tragedy on the same day.
He explains that if the Jewish people didn't consider Aharon's death a tragedy before his death, then the date of Rosh Hodesh Av can be used for another tragedy years later - for the destruction of the first Beit Hamikdash. (Hat tip YUTorah)
We see from here that crying after the tragedy didn't prevent that day from being used again. It's only the prayers of protest to Hashem in advance that would have.
Check out this scary Lazer Beams post. Let us pray to be saved from all troubles which should not come.