Commentary for Taanit 60:12

1 "[the day] on which the deaths ceased in the desert" - As it was taught in a braita, all 40 years that they were in the desert, every Erev Tisha B’av, the proclaimer would go out and say: let each individual go out and dig [his grave]. And every single person would go out and dig for themselves a grave, and sleep in it, in case they would die before they had dug. And the next day, the proclaimer would go out and proclaim: let the living separate themselves from the dead. And every one who had the soul of life would rise and go out [of his grave]. And every year they would do this. And in the 40th year, they did it, and the next morning everyone stood up alive. And when they saw this, they were astonished, and they said, ‘perhaps we made a mistake in calculating the month [and it is not yet Tisha B’Av].’ They returned and slept in their graves each night until the 15th, and since they saw that the moon was full on Tu [B’av] and no one had died among them, they knew that the calculation of the month was in line, and that the 40 years of the decree had been completed, they established that day as a Yom Tov.