To recap history; the fast of Asara b’Teves marks the beginning of the final siege of Jerusalem. On 17 Tamuz the walls were breached; and on 9 Av, the Temple was sacked and destroyed.
Asara b’Teves has a quirk to it in Halacha. The BeHaG, a late Rishon, ruled that the fast on the tenth if Teves is observed on Shabbos, and Friday too. This never occurs with our fixed calendar, but with the fluctuating calendar it could. The same is not true of any other fast, barring Yom Kippur – what is markedly different about Asara b’Teves that it could be observed in Shabbos?
A story is told of a sad old gentleman, one Shabbos afternoon in the city of Psyszcha. Noticing his despondency, R’ Simcha Bunim ambled over to him, and told him that sadness has no place on Shabbos. “Rosh Chodesh and Yom Kippur, Shabbos steps aside. But not for Tisha b’Av!”
Sadness has no place on Shabbos – so again, why does Asara b’Teves have the capacity to override regular Shabbos observance?
The Shulchan Aruch records the law that for certain types of bad dreams, a person can and should fast (if they are bothered by what they saw). Such a fast can be observed even on Shabbos, also overriding regular Shabbos observance. The reason for this is that for such a person, addressing his concerns and fears is his only way of having a peaceful Shabbos.
Dealing with such matters that require resolution is not sadness, and makes perfect sense.
There is a Gemara that states that if a generation fails to see the Temple rebuilt in their days, it is considered to have been destroyed in their days. The Chasam Sofer says that Halachically, the evaluation is very simple; if the Temple existed at that moment, would it continue to? If it is not built yet, it is because it would not last in such an environment.
The last time this evaluation generated a different outcome was Asara b’Teves – the generation failed and the siege began, setting into motion a chain of events. This lends an extra function beyond that of stirring a person to Teshuva, like a regular fast.
It then emerges why it overrides regular Shabbos observance; like the bad dream, the looming cloud disturbs and threatens us. It is a din Torah, a court case. It overrides Shabbos because it is detrimental to our Oneg Shabbos – our concern should be for its construction, may it come quickly.