It is common practice for discussions of Asarah Be-Tevet to refer to R. David Abudarham (14th-century Seville), who rules that were the tenth of Tevet to fall out on Shabbat we would fast on that day as opposed to pushing the fast to Sunday. As a prooftext, Abudarham invokes the verse in Yechezkel (24:2), "be'etzem ha-yom ha-zeh, on this very day." This phrase is taken to mean that Asarah Be-Tevet is intrinsically a day of mourning, and for this reason the fast cannot be delayed. Abudarham concludes by drawing a comparison between Asarah Be-Tevet and Yom Kippur, the only other calendar fast day that is not delayed. The citation from Yechezkel notwithstanding, however, Abudarham's position is perplexing. As R. Joseph Karo notes in his Beit Yosef (O.C. 550), "ve-lo yadati me'ayin lo zeh," "I don't know from where he derived this." Abudarham, in other words, makes a striking halakhic assertion on the basis of little more than a hint from an obscure verse in Yechezkel, hardly basis for a far-reaching halakhic innovation.
The comparison to Yom Kippur, moreover, seems difficult. Self-flagellation on Yom Kippur is a biblical commandment. One who eats or drinks on it is subject to severe punishment, which is why Yom Kippur overrides Shabbat. As such, the comparison to Asarah Be-Tevet seems to miss the mark.
As is often the case, a closer examination of the pertinent biblical narrative offers insight into the subject at hand. In our instance, a deeper appreciation of the book of Yechezkel leads to a fuller appreciation of Asarah Be-Tevet's significance.
Sefer Yechezkel runs 48 chapters long. The book neatly divides into two sections: the first 24 describe the tragedies that will befall Jerusalem and her Temple, followed by visions of consolation. Chapter 24, then, is the coda of the book's first half. A closer examination of the chapter's story reveals that it in fact serves as the fulcrum around which the transition between the book's two sections revolves.
God opens the chapter by instructing the prophet to record "this date, this exact day; for this very day the king of Babylon has laid siege to Jerusalem." God goes on to convey to Yechezkel a devastating prophecy of conflagration and destruction. The navi is shown a pot that has been brought to full boil, and that contains all the ingredients necessary to consume that which is lowered into the pot. The same fate, the prophecy concludes, will befall the Jewish people, which will be set ablaze due to its sins.
In the final section of the chapter, the prophecy takes a surprising turn. Yechezkel is informed that his wife will die (!) but he may not engage in any mourning practices. Similarly, when the Temple is decimated the Jews shall not mourn; instead they are instructed, "you shall be heartsick because of your iniquities and shall moan to one another." Reinforcing the people's change of heart, God concludes the chapter by informing Yechezkel, "On that day... you shall speak and no longer be dumb. So you shall be a portend for them, and they shall know that I am the Lord." The finality of the destruction will cause the nation to recognize that Yechezkel was right all along. They will not mourn for the Temple but for the tragic sins that have led their holy Temple to the brink of ruin.
The tenth of Tevet, in other words, was far more than a key date on the path toward the Temple's destruction. This event also marked a crucial psychological shift in the nation's psyche, as they finally confronted the stark reality of devastation that their sins had wrought. In this sense, it serves as the perfect transition to the consolatory section of Sefer Yechezkel. And as Radak and Metzudat David note, it is for this reason that Yechezkel is instructed to record the tragedy's precise date: when those in Israel hear that Yechezkel, prophesying from Babylonia, has correctly predicted the siege, they will confront incontrovertible evidence of his prophecy's veracity and will begin to change their tune.
It is in this sense that we can understand Abudarham's novel position. The tenth of Tevet possesses dual significance. First, it was on this date that our fate was utterly sealed, as evidenced by the violent vision of the burning cauldron. Second and perhaps more importantly, it was on "this very day" that the process of repentance began. The jarring reality of the siege drove the Jews to a 180 degree turn toward repentance.
In light of the above we can also better appreciate the connection to Yom Kippur. Of all days on the calendar, Yom Kippur provides a unique opportunity to "seize the moment" and dramatically change course. Thus, Abudarham's invocation of "be'etzem ha-yom ha-zeh" highlights precisely this point. On Yom Kippur, as Rambam (Laws of the Fast of the Tenth 2:7) teaches, there is a unique obligation to take advantage of the opportunity afforded by the day and engage in repentance. Thus, the linkage between Yom Kippur and Asarah Be-Tevet lies not only in their shared terminology or external similarities, but in their central teaching.
As it turns out, far from being a obscure source for a surprising halakha, Abudarham's citation of the phrase "on this very day" captures the essence of the fast. From time to time we experience significant hardship or joyous occasions. Asarah Be-Tevet reminds us that when we encounter these opportunities, we have the responsibility - and capacity - to harness these dramatic moments for growth and repentance.