The 10th day of the tenth Jewish month,* Tevet (known in Hebrew as Asarah B'Tevet), which this year coincides with Jan. 10, is observed as a day of fasting, mourning and repentance. We refrain from food and drink from daybreak to nightfall, and add Selichot (penitential prayers) and other special supplements to our prayers. The fast ends at nightfall, or as soon as you see three medium-sized stars in the sky. See your local Jewish calendar or an online one (e.g. chabad.org) for exact times. (For when this fast falls on Fridays, as last year, see the "P.S." below).

What tragic event in Jewish history does the 10th of Tevet commemorate?

For years, G-d had sent His prophets to warn Israel about the impending destruction of Jerusalem and the Holy Temple if they didn’t mend their ways. But they derided the holy men as bearers of “false prophecies of doom,” bent on demoralizing the nation. They even went so far as to kill one of the prophets (Zacharia).

Then it finally happened. On the 10th day of the Jewish month of Tevet, in the year 3336 from Creation (425 BCE), the armies of the Babylonian emperor Nebuchadnezzar laid siege to Jerusalem.

Ever patient, G-d delayed the destruction to give the Jews yet another chance to repent. He repeatedly sent the prophet Jeremiah to admonish His nation, but they foolishly had him imprisoned. Thus, 30 months later, on 9 Tammuz 3338, the Babylonians finally broke through, the city walls were breached (the comemmerative Fast Day became moved to 8 days later on the 17th of the month, because that is the date when the walls were breached in prelude to the destruction of the Second Temple, 490 years later; exactly one month after, on 9 Av of that year -- "Tisha b'Av" -- the Holy Temple was destroyed and the Jewish people were exiled.

The 10th of Tevet is viewed as the beginning of the chain of events that culminated with the destruction of the Temple and the subsequent exiles, something that we have never fully recovered from, because even when the Second Temple was finally built, it never returned to its full glory.

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[From chabad.org. See there for a lot more information and insights.]

ALSO, this Fast Day is the traditonal date for saying Kaddish for Jews whose dates of death are unlnown, including the murdered victims of the bestial Nazis. In Israel it is called Yom HaKaddish HaKlali –“The Day of the Inclusive Kaddish”

* See Exodus 12:2 that one of the 613 commandments is to number the months starting with Nissan, the month of the Exodus and the Passover festival. Interestingly, the first of Nissan is exactly six months before Rosh Hashana, the “Head of the Year.”

P.S. A Rare Occasion

Six specific communal fast days are prescribed by Torah, the three mentioned above plus the "Fast of Gedalia" on the day after Rosh Hashana, the "Fast of Esther" on the day before Purim, and Yom Kippur. Should the three above or the Fast of Gedalia fall on Shabbat (Saturday), the fast is postponed till Sunday; if it falls on Friday, Erev Shabbat, the fast is advanced to the previous day, Thursday, so as to not enter Shabbat fasting.

The other two, Yom Kippur and Tenth of Tevet, if they fall on either Erev Shabbat or Shabbat, the fast must nevertheless be completed on those days, even though it entails intruding into Shabbat -– Yom Kippur because of its supreme importance, and the Tenth of Tevet because a prophetic verse, Ezek. 24:2, emphasizes the importance of the specific date.

Today, the Jewish calendar is fixed so that Yom Kippur does not fall on Friday, nor Tevet 10 on Shabbat. But when the Fast of the Tenth of Tevet falls on Friday, Erev Shabbat (which is relatively rare, but which last happened three years ago for the first time in 7 years), we must fast from daybreak until nightfall. At that moment it is preferable not to break the fast until reciting (or hearing) the Kiddush for Shabbat evening over a cup of wine (or, if necessary, grape juice).

May those of you who fast have a smooth, painless, meaningful experience.

BTW,FYI, in the northern hemisphere, this is the shortest fast of the year.