The Chasam Sofer explains why the 9th of Av was designated and not the 10th? It says ובאו בה פריצים וחיללה, when the blood shedders went inside, they made it mundane. Once they threw fire to the Beis Hamikdash from the outside, it became a house which was non-sacred any more that was burning making the beginning when they started the burning, the time to commemorate.
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