On the 10th day of the month of Tevet, in the year 3336 from Creation (425 BCE), the armies of the Babylonian emperor Nebuchadnezzar laid siege to Jerusalem.
30 months later, on Tamuz 9 (or 17, the very date the walls would be breached when the Second Beis Hamikdash was destroyed), 3338, the city walls were breached, and on 9 Av of that year the Beis Hamikdash was destroyed and the Jewish people were exiled.
Unique among Jewish fasts, 10 Tevet is observed even when it falls on a Friday, though it interferes somewhat with Shabbat preparations.
It is viewed as the beginning of the chain of events that culminated with the destruction of the Beis Hamikdash and the subsequent exiles, something that we have never fully recovered from, because even when the Second Beis Hamikdash was finally built, it never returned to its full glory.
The 10th of Tevet also commemorates two tragic events that occurred close to that date, which were incorporated into the Selichot of 10 Tevet.
The Rebbe recites the Haftarah on Asara B’teves 5748.
This is an audio recording of the Rebbe reciting selichos on the Tenth of Teves. The clip is narrated by Rabbi Shlomo Zarchi who was a yeshiva student at the time.