Why This Fast?
The reason given for fasting on Asarah B’Teves is that it is the day that the wicked Babylonian king Nevuchadnetzar started his siege of Yerushalayim, foreshadowing the beginning of the end of the first Beis Hamikdash, which culminated with its destruction on Tisha B’Av several years later. Therefore, Chazal declared it a public fast, one of four public fast days that memorialize different aspects of the catastrophes and national tragedies associated with the destruction of both Batei HaMikdash.
Three Day Fast?
According to the special Selichos prayers said on the fast, a unique aspect of Asarah B’Teves is that we are actually fasting for two other days of tragedy as well; the 8th and 9th of Teves . In fact, both the Tur and Shulchan Aruch assert that if possible one should try to fast on all three days. Nevertheless, of the three, only Asarah B’Teves was actually mandated as a public fast day.
The 8th of Teves
On the 8th of Teves , King Ptolemy II (285 - 246 B.C.E.) forced 72 sages separately to translate the Torah into Greek (the Septuagint). Although miracles guided their work and all of the sages made the same slight but necessary amendments, nevertheless this work is described as “darkness descending on the world for three days”, as it was now possible for the uneducated to possess a superficial, and frequently flawed understanding of the Torah , as well as providing the masses with a mistaken interpretation of true morality.
The 9th of Teves
Although several decisors write that the reason for fasting on the 9th of Teves is unknown, nonetheless many sources, including the Kol Bo and the Selichos recited on Asarah B’Teves , as well as many later authorities, explain that this is the day on which Ezra HaSofer (as well as possibly his partner Nechemiah ) died. Ezra, the Gadol HaDor at the beginning of the time of the Second Beis HaMikdash , had a tremendous impact upon the nascent returning Jewish community of Eretz Yisrael . He drastically improved the spiritual state of the Jewish people and established many halachic takanos , many of which still apply today. With his passing, the community started sliding from the great spiritual heights Ezra had led them. Additionally, since Ezra was the last of the prophets, his passing signified the end of prophecy.
Other sources attribute fasting on this day to the passings of other specific Tzaddikim on this day, including Shimon HaKalphus and Rav Yosef HaNaggid , or to the birth of ‘ Oso HaIsh ’, the founder of Christianity, in whose name myriads of Jews over the millennia were r”l murdered (see extensive footnote 8). The Sefer HaToda’ah posits that it’s possible that “the darkness descended on the world for three days” alludes to the triple woes of these three days: the 8th, 9th, and 10thof Teves .
Fasting on Friday?
Another exclusive characteristic of Asarah B’Teves is that, as mentioned previously, it is the only fast that can fall out on a Friday. This is fairly interesting as there is a whole debate in the Gemara about how to conduct fasts on a Friday, when we also must take kavod Shabbos into account, implying that it is a common occurrence. However, according to our calendar, a Friday fast is only applicable with Asarah B’Teves , and it happens quite infrequently. The last few times Asarah B’Teves fell out on a Friday were in 1996, 2001, 2010, and 2013. It is next expected to occur in 2020 (5781). After that, 2023 (5784), 2025 (5785), 2034 (5795), and 2037 (5798).