Yes, I know. You aren’t supposed to fast on the Sabbath. But the 9th of Av is still the 9th of Ave no matter what.
This Tisha B’Av is an appropriate time to recall Fast of the Eighth of Tevet, commemorating the disaster of the Torah being translated into Greek, thus “darkness descended on the world” with its abuse.
That fast was rolled together with Fast of the Ninth Tevet in commemoration of the death of Ezra the Scribe, and with Holocaust remembrance for religious Jews, and all three are now part of the Fast of the Tenth of Tevet, which was the start of the Siege of Jerusalem.