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
Our world is a world where a rainbow could be.
At first, there was a world that only received and returned no dividends. Its inhabitants took no ownership. They lived with their Creator’s benevolence, they did what they did with no need for excuses, and eventually died as they died. And that was it.
With the Flood, this world was recreated. The earth was cleansed, the atmosphere purified. It became a world that could take the sunshine that poured in from above and refract it into many colors.
It became a world where a created being could be born, take the soul, body, share of the world and all the sustenance its Maker gave to it, use that, do something with that—and then return it, saying, “See what I have done with that which You gave me!”
And so, G‑d vowed to never destroy the world again. For, if the inhabitants would go wrong, they might always turn around and clean up their own mess.