Asarah B’Teves marks the beginning of the Churban when the Babylonians under Nevuchadnetzer began their siege of Yerushalayim. This siege continued for two years until they breached the city's walls on the 17th of Tamuz, resulting in the destruction of the Beis Hamikdash three weeks later on Tisha B’Av.

Usually, when a fast day falls on Shabbos, we postpone it until after Shabbos, with Yom Kippur being the exception.  Hence, we have the phenomenon of a Ta’anis Nidcheh in Halacha. The Geonim, Rishonim, and Poskim address an interesting opinion, that Asarah B’teves should be considered as Yom Kippur, and that if it falls on Shabbos, we would not postpone it and we would be obligated to fast.  (According to our calendar as designed by Hillel Hazakein, this cannot occur.)

We discuss the essence of the Asarah B’teves fast, and the difference between Asarah B’Teves and the other three fasts that would cause those Geonim (and Rishonim) to consider that one would be obligated to observe this unique fast even on Shabbbos.