The festive day is known as the New Year of Chassidut and the Redemption Holiday.
Rabbi Shneur Zalman of Liadi, called the "Alter (old in Yiddish) Rebbe" by students of Chabad chassidut was imprisoned after being charged with treason, a crime punishable by death.
The Russian authorities forged documents and created a narrative accusing Shneur Zalman of making a pact with Turkey to destroy Russia. He was imprisoned in an island fortress on the Neva River in Petersburg. During his interrogations, he was compelled to present to the Czar's ministers the basic tenets of Judaism and explain various points of Chassidic philosophy and practice. After 52 days, he was exonerated of all charges and released.
In its formative years, the Chassidic movement was the object of strong opposition from the Jewish establishment. Even within the Chassidic community, many of Rabbi Schneur Zalman's contemporaries and colleagues felt that he had "gone to far" in allowing the masses access to the hidden aspects of the Torah.
Rabbi Schneur Zalman regarded his arrest as an earthly echo of a Heavenly indictment against his Torah teachings. He saw his release as signifying his vindication in the Heavenly court and redoubled his efforts, disseminating his teachings on a far broader scale, and with more detailed explanations, than before.
"....G-d has sanctified and magnified His name because even the gentile officials and all the people of the kingdom recognized that 'All this was from G-d; it is all miracles in our eyes' (Psalms 118:23)," he wrote after his release.
Chassidim therefore view the day as a celebration of the victory of the Chassidic approach over its detractors.
The 19th of Kislev did not gain significance only from that event, however. Thirty-six years earlier, in 1772, Rabbi Dov Ber, the "Maggid of Mezeritch" – a student of Chaddism's founder the Ba'al Shem Tov and Schneur Zalman's teacher - passed away. Before his passing, he told the Alter Rebbe, "This day is our festival."
In recent history, Saddam Hussein was captured in an underground hideout on the 19th of Kislev in 2003.
Customary observation of the day includes the giving of extra charity and the setting aside of time to study works of Chassidut, particularly Rabbi Shneur Zalman's magnum opus, the Tanya.