We know that the prohibitions on the Ninth of Av are lifted in the afternoon – and that the fire that destroyed the Beis Hamikdash was started at that time.
But if the destruction began before the 9th of Av, isn’t our mourning greatest when the fire raged, destroying our connection to G-d? In other words, why do we relax prohibitions from the time the fire was lit? Indeed, R’ Yochanan says that he would have declared the 10th of Av to be the day of mourning, because that is when most of the destruction happened.
I would suggest there is a good reason why the Rabbis instituted the 9th, and not the 10th, as the chief day of mourning. The 7th to the 9th were days when the Temple was physically desecrated. These were acts that debased the holiness of G-d’s house, by introducing idol worship, debauchery and perversion. It was lowering G-d’s own house.
But fire is not base, or physical. Instead, fire is one of the core components of serving Hashem, and is a symbol of holiness. The fire of an offering, like the fire of the Menorah and the fire of the burning bush, serves to elevate the physical world into the realm of the spiritual.
In this sense, there was a bittersweet element to the Beis Hamikdash on fire. On the one hand, it was being destroyed. But on the other hand, destruction by fire was at least the addition of energy, of the spiritual plane. The entire Temple was elevated in the act of destruction. And so while we mourn the loss of the Temple, our grief is lessened that its final end was through an aliyah.