Going to Shul on Tisha B'Av

Tosfos comments that we see from the Tosfos HaRav, that Tisha B’Av is comparable to an avel after three days of mourning.

So, in short, the reason we may go to shul on Tisha B’Av is because we treat it like someone who is an avel after three days of mourning. (See further, the Maharsha and Korban Nesanel, Perek 3, siman 37)

R’ Shlomah Zalman Auerbach zt”l many times on Tisha B’Av would bring this Tosfos to raise people’s awareness of just how serious the aveilus of Tisha B’Av actually is.