Asarah B'Tevet is a minor fast day in the Jewish calendar, observed on the 10th day of the month of Tevet. It commemorates the beginning of the siege of Jerusalem by the Babylonian king Nebuchadnezzar in 588 BCE, which ultimately led to the destruction of the First Temple and the exile of the Jewish people. On this day, Jews fast from dawn until nightfall, refraining from eating and drinking. It serves as a time for reflection and mourning over the tragedies that befell the Jewish people, particularly the loss of their homeland and the Temple.