Translated by Sichos in English:
1. Tonight is the eve of the Tenth of Teves, one of the four fasts instituted by the Rabbis in connection with the destruction of the Beis HaMikdash. The Tenth of Teves possesses a dimension which makes it more severe than the other Rabbinical fasts, even Tishah BeAv.1 For all the other fasts are postponed if they fall on Shabbos. In contrast, were the Tenth of Teves to fall on Shabbos, it would be necessary to fast on that day, for in regard to this fast it is written, “On the essence of this day….”
(According to the fixed calendar we follow at present, this is impossible. When, however, the calendar was established through the testimony of witnesses,2 it was possible for the Tenth of Teves to fall on the Sabbath.)
This concept is also relevant to us at present. For a fast day is, as the Alter Rebbe explains in Iggeres HaTeshuvah, “a day of will.” Since the obligation to fast on the Tenth of Teves is stronger than on other fasts, it can be understood that the element of being “a day of will” is also stronger than other fasts, even than Tishah BeAv which, as explained on previous occasions, is significant because it is the birthday of the Mashiach.3
The strength — both of the obligation to fast and the positive influences — of the Tenth of Teves stems from the fact that it commemorates the first of the tragedies associated with the destruction of the Beis HaMikdash. On this day, Nebuchadnezzarlaid siege to Jerusalem.
Thus this date begins the process of destruction. It is well known that the beginning of any process contains more power than the subsequent stages and for this reason, there is added power to the Tenth of Teves.
The connection between the Tenth of Teves and fasting is also reflected in the very event which transpired on that day. Since the Babylonians laid siege to Jerusalem, its inhabitants were unable to go out and procure food supplies. Although the Babylonians did not enter the city on this day, they merely camped around its wall, this was serious enough to cause hunger. The ultimate intent, however, was that this would motivate the Jews to teshuvah, and thus ward off the subsequent tragedies which culminated in the Beis HaMikdash’s destruction.
Just as the Tenth of Teves began the process leading to the Beis HaMikdash’s destruction, it is also appropriate that this “day of will” be used to hasten the coming of the Redemption. This is particularly true since, as the Zohar explains, the Jews are on the level of tzaddikim. As such they are prepared to turn to G‑d in teshuvah.
Teshuvah is an instantaneous process. For at every moment, a person can turn to G‑d. We see an expression of this concept in Torah law: If a man consecrates a woman as his wife on the condition that he is totally righteous, the act is valid even if he was known to be utterly wicked beforehand. Why? We assume that he repented at that moment.
This process of turning to G‑d in teshuvah will speed the Redemption, for as the Zohar writes: Were a single tzaddik to turn to G‑d in teshuvah, the Redemption would come. Similarly, the Redemption will be hastened by the distribution of money to be given to tzedakah.
And in the immediate future, we will proceed “with our youth and with our elders… with our sons and with our daughters,” “on the clouds of heaven” to the city of Jerusalem. There will be no siege around the city’s walls. On the contrary, the city will expand and grow as will all matters associated with Jerusalem, including and in particular, the Beis HaMikdash, “the Sanctuary of G‑d established by Your hands.”
In other ways, Tishah BeAv is more severe. For on Tishah BeAv, the fast begins from the eve of the day, while on the Tenth of Teves and the other Rabbinic fasts, the fast begins only in the morning.
Indeed, it would have been appropriate that all the fasts last for a full day. Nevertheless out of G‑d’s compassion for His people, He takes their health into consideration and limits the fasts other than Tishah BeAv to the daytime hours.