Here's my chidush for Tu B'av:
In Talmud Yerushalmi Masechet Taanit, chapter 4, halachah 7, we have an opinion as to why we celebrate Tu B'av. Translation is my own:
Rabbi Abun says that in it (Tu B'av), the digging was cancelled. And Rabbi Levi says (explaining the statement of Rabbi Abun): Every Tishah B'av Moshe would send out an announcement in the whole camp and say "Go out and dig! Go out and dig!" And they would go out and dig themselves graves and sleep in them, and in the morning they would stand up and find themselves missing 15,000 and change (the word is protrot), and in the last year they did thus and found themselves whole (i.e. no one died). They said "perhaps we made a mistake in our calculations, and thus on the 10th, 11th, 12th, 13th, 14th, and 15th. And because the moon was full they said that it appears that HaKadosh Baruch Hu has nullified that harsh decree from us, and they stood up and made a Yom Tov celebration.
The story is difficult. How could it be that Moshe tells them to go lie down and that no one passed away the last year? Korban HaEidah and Pnei Moshe explain that the Gemara is precise. Moshe didn't say to go out the last year, but "they", meaning the people, did so at their own volition, without an order from Moshe.
These people were in essence part of the collective group of people that did not have faith in Hashem when the Ten Spies came back from Eretz Yisrael. The next day some of them went to battle against the Emori and Amaleki, against the command of Hashem via Moshe. They were punished for lack of faith and not listening to Moshe.
Now here they are, essentially being freed of the punishment for this sin. Moshe didn't tell them to dig their graves. So what do they do? They dug their graves!
But even after that, they stay in the graves straight through the 15th of Av! Did they not know that every year the punishment was on the 9th of Av? The Gemara says that they got up on the 15th of Av, because only then the moon was full.
But the 9th of Av is the 9th of Av. The 9th of Av is the ninth day starting from Rosh Chodesh. There is no doubt as to the date. Even if for some reason they think they counted wrong, all they had to do was walk over to the Sanhedrin and ask them the date, and that was the end of their doubt. But they didn't. Only on the 15th did they feel confident enough to celebrate. On the 15th, they had non-Torah confirmation that they were safe.
Shaar HaGilgulim Hakdamah 20:
..also then, in the last generation, the entire Generation of the Desert, including the Erev Rav, will be reincarnated...
This is the test of our generation. Faith. The Generation of the Desert, according to their level, had open miracles to prove Hashem's Hand in this world, and yet they couldn't translate their faith in to action. So too we see in our generation. Countless miracles occurred since 1948. However we have seen a consistent pattern, especially since the early 1990's, of running away from the consequences of these miracles.
It is my prayer that we speedily move towards a complete rectification of this fault and we begin to view our faith as a practical matter that is to be reflected in our daily and national lives.