This fast is unusual because it is the only one that can fall on a Friday, as it does this year. In this case, the fast begins at dawn and, instead of ending at nightfall, is instead only broken once kiddush is recited on Friday night, the Shabbat eve.
Known in Hebrew as Asarah B'Tevet, (10th of Tevet), the fast was originally established to commemorate the beginning of the siege of Jerusalem by the Babylonian emperor Nebuchadnezzar in 425 BCE. The siege was "the beginning of the end" and continued for 30 months until the walls of Jerusalem were breached, eventually leading to the destruction of the First Temple on the 9th of the Hebrew month of Av in 586 BCE, a full 24-hour fast done in the summer.
The fast of the 10th of Tevet eventually came to commemorate the translating of the Torah into Greek – considered one of the darkest days in Jewish history – and the passing of Ezra the Scribe, the leader who brought back the Jewish people to the Land of Israel at the end of the Babylonian exile.
Asarah B'Tevet has also been accepted as a day to say kaddish for those killed in the Holocaust.
There are three other minor fasts (meaning they last from daybreak to nightfall) observed by Jews: the 17th of Tammuz three weeks before the 9th of Av, the fast of Gedaliah after Rosh Hashanah and the Fast of Esther before Purim. The two major fasts (meaning they last from before nightfall to nightfall the next day) are the 9th of Av, when the Temples were destroyed, and Yom Kippur, the Day of Atonement.
Chief Rabbi of Israel Rabbi David Lau ruled earlier this week that those who are infected with the coronavirus cannot fast on the 10th of Tevet, even if they are asymptomatic. Those who are in quarantine and feel well should fast; those who are in quarantine who do not feel well should not fast.
Lau also instructed worshippers to replace the word "withhold" in the verse of the Avinu Malkeinu prayer pleading "Avinu Malkeinu mena mageifa mi'nahalateinu" (Our Father, Our King withhold the plague from Your inheritance) with the word stop: "...atzor mageifa mi'nahalateinu" (stop the plague from Your inheritance).
Lau added: "May the Lord fulfill the words of the prophet Zechariah: 'and the fast of the tenth month [of Tevet] shall be for joy and gladness, happy festivals for the House of Judah; but love truth and peace.' (Zechariah 8:19)" in his ruling published earlier this week.
Rabbi Moses ben Maimon, known as the Rambam or Maimonides, wrote that fast days serve to remind us of "our wicked conduct and that of our ancestors, which resembles our present conduct and therefore brought these calamities upon them and upon us." (Mishneh Torah, Fasts 5:1) The goal of the reminder is so that "we will repent and improve [our conduct]," according to the major 12th century medieval Torah scholar.
Fast Start and End Times: (According to MyZmanim. The fast should be broken only after kiddush is recited on the Shabbat eve. There are varying customs)
Jerusalem
Start: 5:19 AM
End: 5:22 PM
Tel Aviv
Start: 5:21 AM
End: 5:23 PM
Haifa
Start: 5:21 AM
End: 5:21 PM
Eilat
Start: 5:16 AM
End: 5:27 PM
Beersheba
Start: 5:20 AM
End: 5:25 PM
New York
Start: 5:50 AM
End: 5:20 PM
Los Angeles
Start: 5:36 AM
End: 5:32 PM