On the 10th of Tevet of the year 3336 from Creation (425 BCE), the armies of the Babylonian emperor Nebuchadnezzar laid siege to Jerusalem. Thirty months later -- on Tammuz 17, 3338 -- the city walls were breached, and on 9 Av of that year, the Holy Temple was destroyed. The Jewish people were exiled to Babylonia for 70 years.
Link: Asarah B'Tevet
Tevet 10 is observed as a day of fasting, mourning and repentance, in remembrance of the siege of Jerusalem. We refrain from food and drink from dawn to nightfall, and add the Selichot and other special supplements to our prayers. (More recently, Tevet 10 was chosen to also serve as a "general kaddish day" for the victims of the Holocaust, many of whose day of martyrdom is unknown.)
Links:
Learn about Tevet 10
Essays and Stories on the Holocaust
There’s nothing that matches the joy when a lost child returns home. And you are that child.
Because your soul began within G‑d.
And she descended into this world to inhabit a body and live as though a stranger to the heavens, captive of a futile dream, journeying upon many winding and perilous roads…
Until waking up one day and realizing that none of these lead back home.
And now, every day, with every prayer of your heart, with every word of Torah upon your lips, with every mitzvah and good deed you fulfill,
You are coming home.
There can be no greater joy—above and below.