The Navi Zechariah (8:19) promises that four fast days will in the future be transformed to holidays. The fast of the tenth month, namely Asarah B' Teves, the day which commemorates the siege that Nebuchadnezzar, king of Babylonia lay to Jerusalem (586 BCE), which marked the beginning of the end of the first Bais HaMikdash, will in the future become a day of celebration.
The Avudraham in his commentary on the siddur writes a fascinating halacha. He says that the Navi Yechezkel (24:2), who instructed the Jewish community in Bavel where he was exiled from Eretz Yisrael, to write "b'etzem hayom hazeh"- to note with precision and surety that on this day- the Tenth of Teves- the siege was placed around Jerusalem. Because the Navi uses this expression several times we are to learn the importance of fasting precisely on that day. It is for this reason that Asarah B'Teves is the only fast that can fall on a Friday and indeed we fast on Erev Shabbos. The remarkable teaching of the Avudraham is that were this fast to occur on a Shabbos, we would fast on Shabbos as well.
At first glance this seems rather startling. True, the Rabbis orchestrated the calendar that the tenth of Teves cannot occur on a Shabbos. But what is it about this day, wherein lies its significance that it would join Yom Kippur which is the only fast that the Jewish people fast on Shabbos?
The Chasam Sofer zt"l in his commentary on the slichos for Asarah B'Teves provides an insightful explanation of the Avudraham. He reminds us that what transpires here on earth is a reflection of what is transpiring in heaven. The siege of Bavel was indeed to serve as a wake-up call to the Jewish nation to reflect upon the possibility that they might lose the Bais HaMikdash. On high, the Heavenly Court was called into session to determine if the Bais HaMikdash would indeed be destroyed. Moreover, annually, writes the Chasam Sofer zt"l, on every Asarah B'Teves the Bais Din Shel Maalah is convened to decide if the third Bais HaMikdash will be built this year or Heaven Forbid postponed.
There are two models of mourning. The first is called aveilus yeshana- a mourning for a historical phenomenon that affected us adversely in the past. Tisha B'Av is just such an occurrence. For an aveilus yeshana we do not fast on Shabbos. Thus when Tisha B'Av falls on Shabbos we fast on Sunday. Those who have the practice of fasting on a yahrzeit do not fast on a Shabbos, as that too is commemorating an aveilus yeshana.
However, there is also a model of aveilus chadasha, meaning a present or contemporary mourning. If one has a chalom rah (a disturbing dream) on Friday night, the halacha (Orach Chaim 220:5) allows the individual to fast on Shabbos since for them there is an immediate crisis and fasting would often be a greater personal oneg (pleasure) than partaking of the Shabbos meals. Similarly, with the insight of the Chasam Sofer, the Heavenly Deliberations on Asarah B'Teves designates this fast as not only historical and commemorative, but creating a national crisis, inspiring us to storm the Heavenly Court with our fasting and prayers.
The urgency and immediacy of Asarah B'Teves may further be gleaned from the teaching of the Yerushalmi (Yuma 1:1) that "any generation that did not build the Bais HaMikdash in its day- it is as if it were destroyed in its generation". The Sfas Emes explains that there have been many generations of righteous individuals such as Tanaiim and Amoraim, the chachamim of the Talmud and still the Temple has not been built. He explains, based upon Tehillim (147:2) "boneh Yerushalayim Hashem - Hashem builds Jerusalem", meaning it is a process and every generation must contribute its share of building the third Bais HaMikdash by proving its worthiness of the Divine dwelling literally in its midst. Asarah B'Teves is our appearance before the Heavenly Court, hence we would fast on a Shabbos.
Finally, the Seder Hadoros sees Nebuchadnezzar as the gilgul and direct descendant of Nimrod. Nimrod led the construction of migdal Bavel, which is understood by Chazal (Tanchuma Noach 24) as a direct attack on G-d. The goal was (Beraishis 11:4) "let us make a name for ourselves", i.e. place man at the center of the universe, not any Divine Being. He attempts to dominate the public mindset by casting Avraham, his vocal opponent, into the fire. Avraham, as we know, is miraculously saved.
Nimrod's direct descendant, Nebuchadnezzar, likewise wishes to destroy the Bais HaMikdash. The daily functioning of the Temple accompanied by the ten miracles (as found in Avos 5:7) proclaimed to the world the existence and directorship of Hashem over all. G-d was recognized as the center of the world. Like Nimrod, this was challenged by Nebuchadnezar. He too cast Chananya, Mishael and Azariah into the furnace and they too, loyal descendants of Avraham, were miraculously saved. He too wanted to place man at the center of the universe.
Asarah B' Teves thus invites all proud descendants of Avraham, and the many myriads throughout Jewish history who sacrificed their lives for the sake of Hashem, to bolster their living a life of Kiddush Hashem- sanctifying G-d's name , thereby contributing their share to the building of the Third Temple. Perhaps in contrast to the length of the fast, a day fast as opposed to 25 hours, Asarah B'Teves is called a minor fast, but in reality we know better.