This Thursday we commemorate the attack on Jerusalem by the Babylonian King, Nebuchadnezzar, in 426 BCE. The fast  begins at 6:08 AM and ends at 5:28PM.
The fast is a refraining of eating and drinking, and has none of the other stringencies of Tisha B’Av and Yom Kippur (washing, using cosmetics, wearing leather shoes and marital relations are permitted). Pregnant and nursing women, the infirm, and children are exempt from the fast. Please call or email me if you have any questions about the fast, or issues about fasting.
In Israel, this day commemorates the deaths of those who perished during the Holocaust on an unknown date, and kaddish is recited.

We first learn about the historical significance of the day in the Bible, in the Second Book of Kings (Melachim 2):
“Zedekiah rebelled against the king of Babylon. And in the ninth year of his reign, on the 10th day of the 10th month Nebuchadnezzar moved against Jerusalem with his whole army. He besieged it; and they built towers against it all around. The city continued in a state of siege until the 11th year of King Zedekiah.  On the ninth of the month famine was intense in the city, the people had no bread, and the city was breached.‘”
( II Kings, 25:1-3)
The date is memorialized in the Book of Ezekiel, and the tradition to fast on this day is written in the Book of Zecharia, 8:19.
Because of the explicit mention of the date our sages declared that this fast, like Yom Kippur, could take place on Shabbat itself. The formation of the fixed calendar ensured that this would never actually happen (nor on Mondays..), but the fast does fall on Fridays 20 percent of the time.

Why fast at all?
The Rambam (Maimonides) taught that our spiritual shortcomings were the ultimate cause of all national calamity. He writes:
“There are days in which all the people of Israel fast to repent the misfortunes which befell them. The fasting will serve as a reminder of our bad deeds and the deeds of our fathers which have caused us hard times. Remembering our misguided ways gives us the opportunity to be better people…“  (Maimonides, Mishneh Torah, Hilchot Taaniot, Chapter 5, 1).

Ultimately, at the time of our redemption, these days will all be days of great joy.
May that time come speedily, and in our own lifetime!
Wishing you a tzom kal, an easy fast, and a Shabbat Shalom.
Rabbi Greg