"Legend also connects Tu B’Av to another kind of harvest. Tu B’Av always comes at the full moon of the month of Av. According to rabbinic legend, when the Israelites were wandering in the wilderness, the former slaves were doomed by God to die before reaching the land of Israel. Every year on the ninth of Av, the Israelites would dig graves, lie down in them, and spend the night. In the morning, the people would arise and count themselves to see who had died that year. This weird ritual seems to represent the randomness and scariness of mortality. The story encourages us to meditate on our own death, just as if we were lying in our own grave.
"The story goes that in the fortieth year of wandering, the ritual was enacted, but no one died. Thinking they had miscalculated the calendar, the people slept in their graves a second night, then a third, then a fourth. On the seventh night, Tu B’Av, when the full moon came out, the people knew the decree had ended. They understood that all of them would be able to enter the Promised Land. The time of death and stagnation was over, and the time of life had begun.[4] It was truly a Day of the Breaking of the Axe—a day when mortality no longer held sway.
"R. Levi said: On every eve of the 9th of Av (during the 40 years when the Israelites wandered in the wilderness) Moses used to send a herald through the camp and announce: Go out to dig graves. They would go out and dig graves and sleep in them. In the morning, he would send a herald and say: Separate the dead from the living.” They would arise and find their number diminished. In the last of the forty years, they did this but found themselves undiminished. They said; we must have made a mistake in counting. They did the same thing on the tenth, eleventh, twelfth, thirteenth, fourteenth, and fifteenth, but still no one died. When the moon was full, they said; it seems that the Holy One has annulled the decree from all of us, so they made the fifteenth a holiday. —Lamentations Rabbah, Prologue 33"