Asarah B'Tevet is a minor fast day observed on the 10th day of the Hebrew month of Tevet, typically falling in December or January.

This day involves fasting, mourning, and repentance, accompanied by prayers. Its significance lies in commemorating the onset of the siege of Jerusalem by King Nebuchadnezzar of Babylonia in 586 B.C.E. The siege extended for 30 months, concluding when the walls of Jerusalem were breached.