Halakhah do Królów II 25:1

And they are four days: The third day of Tishrei, on which Gedaliah ben Achikam was killed and the remaining coal of Israel was extinguished and this was a cause for the increase of the woes. The tenth of Tevet, on which the evil Nevuchadnetsar approached Jerusalem, as it is written in II Kings 25:1 and in Jeremiah 52:4, "in the ninth year of [the rule of Tsidkiyahu] on the tenth month on the tenth of the month, Nevuchadnetsar and all of his army came upon Jerusalem." And the Holy One, blessed be He, revealed this to Yechezkel - who was in Babylonia with the exile of Yochaniah - in a prophecy on that same day, as it is written (Ezekiel 24:1-2), "And the word of God was upon me in the ninth year on the tenth month on the tenth day, saying, 'Son of man, write for yourself the name of this day - of this very day - [as] the king of Babylonia approached Jerusalem on this very day.'" Which is to say that He commanded him to write the name of the day, which is the name of the month and also the specific day, and he wrote, "on the tenth of the tenth month," and the prophet did not know for what [reason] He commanded him to write this. And after he wrote it, He revealed to him, "You should know 'the king of Babylonia approached Jerusalem on this very day.'" And through this they all saw the true prophecy, as that day and that time were revealed to the prophet in Babylonia. Since, the exact [time] of the day is called "that very day," as per chatsot hayom (midday). And [this is] because there were then false prophets who fooled Israel, like Chananiah ben Azur and his colleagues, who prophesied that the Temple would not be destroyed and that Israel would not be exiled from their land, and [so] they strengthened the hands of the sinners. And Yirmiah - who rebuked them that they should return to God, and if not, the Temple would be destroyed and Israel would be exiled - they placed in the courtyard of the guardhouse and put him in a pit of mud, as is explained in Jeremiah 38. Hence, God informed Yechezkel the prophet in Babylonia that Yirmiah was a true prophet, and he prophesied in Babaylonia all that was happening in the Land of Israel, and that the Tenth of Tevet was the beginning of all the woes. Therefore, they established it as a fast day for the generations. And it is found in the name of the early ones, that if the Tenth of Tevet would come out on Shabbat, it would push off the Shabbat, as it is written about it, "on that very day," like Yom Kippur (Beit Yosef, Section 550 in the name of Abudarham). But we do not hold like this, as will be explained.