I know this will be controversial, but….
Ittisa and I recently had a discussion (DMC) regarding the Holocaust. I was adamant and she agrees, that as the Torah says:
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All punishments given to Klal Yisroel are rooted in the Chet HaEigel. We (as a Klal) have not rectified and/or have done proper Teshuva for that Avairah and its causes, so we continue to be punished.
A significant problem (IMHO) is that since the punishment of Churban Europe was so great, it (many times) overshadows (at this point in our history) the Churban HaBayis, Chas V’Shalom. V’haraya, the presentations regarding Churban on Tisha B’Av are that of Churban Europe, not Churban HaBayis. Perhaps for that reason alone we should have a separate day, so that the Churban HaBayis could be mourned properly. (As a side point, presentations pushing Kiruv or Shmiras HaLashon are wonderful, but are NOT the point of Tisha B’av, which is a day of mourning regarding the destruction of the Batei Mikdash).
That being said, IMHO (and I am in no way a “gadol”) the appropriate day for mourning Churban Europe would be the 17th of Tammuz, which is the day of the Chet HaEigel. We would then remember how our Chataim cause us to be punished until this day, and how we should due Teshuva. Three weeks later, we would then be able to appropriately mourn individually for the loss of the Batei HaMikdash.
Once again, I’m not a Gadol, but those are my thoughts.