Asarah B’Teves is the only fast day that can fall out on a Friday — as it does this year. What is even more striking and significant is the teaching of the Abudraham, that, were Asarah B’Teves to fall out on Shabbos, it would have the same status asYom Kippur in that Klal Yisrael would be required to fast.
In contrast, Tisha B’Av, which falls out fairly frequently on a Shabbos, is always postponed to Sunday.
At first glance this seems puzzling. On Asarah b’Teves, Nevuchadnetzar, the Babylonian emperor, began the siege of Yerushalayim. It was on Tishah B’Av that the Bais Hamikdash was actually destroyed. One would have thought that if one doesn’t fast on Tisha B’av that falls on Shabbos, one certainly doesn’t fast on Asarah B’Teves.
The Chasam Sofer gives a very powerful explanation:
The halachah is that one doesn’t fast on Shabbos about something that happened in the past, such as fasting on the yahrtzeit of a parent. One is permitted, however, to fast on Shabbos about a bad dream that warns about a gezeirah in the future, for in such a case the oneg of abolishing the decree against the person through fasting supersedes the oneg of eating.
When Nevuchadnetzer laid siege to Yerushalayim on the Tenth of Teves, the Beis Din Shel Maalah also sat and decided whether the Bais Hamikdash should be destroyed.
Chazal teach us that in every generation in which the Bais Hamikdash is not rebuilt, it is considered as if it was destroyed.
Each year on this day, the Beis Din Shel Maalah judges whether the Bais Hamikdash should be rebuilt or R”l, “destroyed” again.
Therefore this fast isn’t about the past but about the future, and even on Shabbos we would be required to fast.
May the Ribbono shel Olam listen to our tefillos, and may we merit the Geulah speedily in our days.