December 5, 2007. It's the Annual Day of the Ninja. As popular as these masked men and women have become you might forget that in feudal Japan, ninjas were real and really sneaky. Under cover of darkness, dressed in black, ninjas were hired assassins and spies. So on this day you can dress like a ninja; at the mall, creep like a ninja; in the office, ask a ninja; and online, for one day only, ninjas go public.