Jubilee is often used to refer to the celebration of a particular anniversary of an event, usually denoting the 25th, 40th, 50th, 60th, and the 70th anniversary. The term comes from the Hebrew bible, initially concerning a recurring religious observance involving a set number of years, that notably involved freeing of debt slaves. Emperors of ancient Rome customarily marked anniversaries of their rule with celebrations, although they did not use the term, jubilee. Nonetheless, the term came into English usage from the bible, together with customary celebration of a reign, and is now often used to denote the celebrations associated with the reign of monarchs after a milestone number of years have passed. Source: Wikipedia (en)