Interesting, but the mourning day mentioned seems more related to Asara B’Tevet, related to the siege of Jerusalem. Maybe someone at some point mixed things up.
Aserah b’Teves actually combines at least three commemorations: The translation of the Torah into Greek (8 Tevet), the yartzheit of Ezra the Scribe (9 Tevet), and the beginning of the siege of Jerusalem (10 Tevet).