But that same source indicates that the twin tragedies of Tammuz and Av far predate the destruction of the Temple, or even its construction: to the Sin of the Golden Calf during the Israelites' first Tammuz in the desert and the Sin of the Spies thirteen months later, respectively.
This brings us to a famous question: what makes ten of the twelve Spies sent by Moses defame the land? Their bad report when they return on 8 Av spells disaster for their entire generation, but forty days earlier, as they set out, we are told "They were all men who were heads of the Israelites" (13:3) -- terminology denoting conspicuous virtue, according to the Midrash (Tanchuma 4). So what happened? When and why do they go bad?
Halfway through their mission would have been day twenty. Counting back from 8 Av, that would be... 17 Tammuz. The first anniversary of the Sin of the Golden Calf. The yahrtzeit of thousands of Israelites.
This is the part of the Golden Calf tale that we usually ignore, but it's quite brutal (Exodus 32:26-29):
So he stood at the entrance to the camp and said, “Whoever is for the Lord, come to me.” And all the Levites rallied to him. Then he said to them, “This is what the Lord, the God of Israel, says: ‘Each man strap a sword to his side. Go back and forth through the camp from one end to the other, each killing his brother and friend and neighbor.’” The Levites did as Moses commanded, and that day about three thousand of the people died. Then Moses said, “You have been set apart to the Lord today, for you were against your own sons and brothers, and he has blessed you this day.”As the Talmud (Yoma 66b) notes, this death toll only takes into account those who were killed directly, by the sword. Many more die from drinking the water into which the Golden Calf had been ground (ibid. v. 20) and still more the from the ensuing plague (v. 35): "And the Lord struck the people with a plague because of what they did with the calf Aaron had made."
Marking the first anniversary of that solemn occasion on a scouting mission, far away from family and friends, would have been extremely difficult. What makes it worse is the fact that the tribe wielding the executioner's axe is conspicuously absent. No Levite goes on this mission. Nevertheless, they are supposed to report back to Moses and Aaron, the latter of whom made the Calf and the former of whom ordered their loved ones' deaths for worshiping it. Nor is the situation improved by the two loyalists among the group. Joshua is Moses' aide-de-camp, while Caleb's grandfather Hur was a strong ally. Is it any wonder that the ten Spies who have no reason to be loyal to Moses, who have every reason to defy him, succumb to their post-Tammuz stress disorder?
Trauma begets trauma, on the individual and the national level. It is only natural for Tammuz to give way to Av. To break the cycle of tragedy takes uncommon courage, but that is the only way we can put an end to our mourning.