From BnaiDavid.com:
Friday, December 17th will be
the Fast of the 10th of Tevet
The fast of the 10th of Tevet commemorates the day on which the Babylonians laid siege to Jerusalem, leading to its eventual destruction in 586 B.C.E. This fast (along with the 17th of Tamuz, Tisha B’Av and Tzom Gedalia) was actually suspended during the Second Temple period, but was volitionally reaccepted by the community of Israel at some point following the second destruction in 70 C.E. This was motivated by the general state of physical insecurity that the Jewish people faces on a continuous basis. As long as this unfortunate state of affairs continues to exist, these fasts are binding upon of us. Women who are pregnant and nursing are exempt, though the
custom is that they make the effort to fast as well.
We mark the 10th of Tevet with special inclusions in davening. At Shacharit (6:30 AM) we include Selichot, Avinu Malkeynu and Torah reading, and at Mincha (4:15 PM) we include a Haftarah reading as well.
Though it strikes us as unusual to be fasting on a Friday, the 10th of Tevet falls on Friday with some regularity. The fast does not end at candle lighting, rather, following the usual pattern, ends at nightfall (see times below).
As is true whenever we fast, we make a point a point to recall the words of Yishyahu HaNavi concerning the ultimate meaning of a fast day:
Behold, this is the fast that I esteem precious:
Loosen the chains of wickedness,
undo the bonds of oppression,
Let the crushed go free, break all yokes of tyranny!
Share your food with the hungry,
take the poor to your home,
Clothe the naked when you see them,
never turn from your fellow.
Then shall your light dawn, your healing shall come soon;
Your triumph shall go before you,
the Lord’s glory backing you.
The Lord will answer you when you call,
saying “Here I am.”
Friday – Fast begins: 5:33 AM
Shacharit – 6:30 AM
Mincha – 4:15 PM
Fast ends: 5:14 PM