by Rabbi Yehudah Prero. The first mention that we see of the tenth day of Teves as a fast day appears in Zecharia 8:19, where the verse mentions the “fast of the tenth month…” As the tenth month counting from Nissan (which is referred to as the first month) is Teves, the fast of the “tenth month” is the Fast of the Tenth of Teves.
The Aruch HaShulchan (Orech Chayim 549) gives us an understanding of why this day is significant. He writes that on the tenth day of Teves, the wicked Nevuchadnezzar, the king of Bavel, laid siege to the city of Jerusalem. This event is mentioned in the second book of Melachim (Kings) (25:1), where we are told that “And it came to pass in the ninth year of his reign, in the tenth month, in the tenth day of the month, that Nevuchadnezzar, king of Bavel came, with all his troops, upon Jerusalem….” At the time of this siege, there were already Jews who were in exile in Bavel, where the prophet Yechezkel (Ezekiel) lived. Hashem came to Yechezkel in a prophecy, to inform him of the siege of Jerusalem. This prophecy is related to us in the Book of Yechezkel (24:1): “And in the ninth year, in the tenth month, on the tenth day of the month, the word of Hashem came to me, saying ‘Son of man, write the name of the day, of this same day – the king of Bavel has laid siege on Jerusalem on this very day….'”
What was G-d commanding Yechezkel to write? The Aruch HaShulchan explains that G-d wanted Yechezkel to write the month and the day of the month on which this prophecy was being received. Why? Because on that very day, the siege of Jerusalem was taking place. Why was this event so significant that Yechezkel’s notification of it came through prophecy, and the date had to be noted? At that time in Bavel, there were false prophets who spread the word that the Temple in Jerusalem would never be destroyed and that the Jewish nation would never be exiled from the land of Israel. While these “prophets” spread their message of falsehood, the true prophets of G-d, namely Yirmiya (Jeremiah) exhorted the nation of Israel to repent and turn from their evil ways. He warned that the consequences of the nation remaining sinful were dire – the Temple would indeed be destroyed and the Jewish nation would indeed be exiled from the land of Israel. However, the nation did not heed the warning of Yirmiya, and they remained unrepentant and steeped in their sinful ways.
When the siege of Jerusalem finally occurred, as the true prophets of G-d had warned, G-d wanted the people to immediately get the message that the prophet Yirmiya was truthful and correct, and that all of his prophecies were the truth. Therefore, Hashem came to Yechezkel on the day that the siege took place and related to him the day’s events. All of the people in Bavel would immediately hear from Yechezkel about the tragic event, the event that the “prophets” they listened to said would never happen. The people of Bavel would see that they erred by not repenting and by ignoring the word of G-d. Yechezkel was therefore told by Hashem to make note of this date, so all would recognize G-d’s true messengers, know of the events and the of the forthcoming consequences which they thought would never happen.
The Aruch HaShulchan concludes that we fast on this day because it marks the beginning of our sorrows – the first event in a chain which resulted in the eventual destruction of Jerusalem and the Temple, and the exile of the nation of Israel. In the event that it were possible for this day to fall out on Shabbos (which it can not, because of our calendar system), there are authorities which said that we would still fast, although fasting on the Shabbos day is forbidden. Why would we nevertheless fast? We would fast because the words used by G-d to describe the events to the prophet Yechezkel were the same words used in conjunction with the description of Yom Kippur, the holiest day of the year, on which we fast even if the day falls out on the Shabbos: the words “On this very day” “B’etzem hayom hazeh.” [Torah.org]