Sleeping in Graves on Tishah B'Av in the Desert
Each Tishah B'Av in the desert, the generation of the spies slept in graves; in the 40th year they awoke alive and, by Tu B'Av, knew the decree had ended.
Hashem had decreed that those men who were between the ages of 20 and 60 at the time of the sin of the meraglim would perish in the desert and not enter into Eretz Yisrael. Throughout their travels in the desert, on erev Tishah B’Av those Yidden included in this harsh decree would dig a grave. They would sleep in their graves that night. In the morning, the people would see who had died that night and cover them.
The Midrash relates that some 15,000 people died every year, while the rest merited to live at least another year. In the 40th year, those who assumed that they were included in the decree went to sleep in their graves on Tishah B’Av night and, to their surprise, woke up the next day. Thinking they had made a mistake with the date, they slept in their graves for four days. But with the full moon showing on the 15th, they knew with certainty that they had not erred. Only then did they know they were allowed to live.