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
Perhaps a person will say,
“I cannot fathom an infinite Creator, so why should I attempt to do so?”
“And why should I attempt to awaken my heart to show Him love? What can the love of this puny creature provide Him?”
“So I will just serve Him in complete surrender, doing that which is to be done, connecting to a will and desire far beyond my own.”
Such a person may be wise, but he is wrong.
Yes, it is absurd, but G‑d desires to be grasped by your human mind.
Yes, it is inconceivable, but He desires to find a home within the innermost chamber of your heart, no matter how tiny that place may be.
For this He created you, so that He could dwell within your world.
Serve G‑d with all your heart and all your mind—because that too is His unfathomable, unbounded desire.