In 2003, the creators of Ninja Burger (parody website) declared that December 5 would be celebrated as Day of the Ninja. On this day, people are encouraged to dress as ninja, engage in ninja-related activities, and spread information on ninja online.
December 5 was originally chosen because December 5, 2003 marked the release of Tom Cruise's film The Last Samurai (which featured a scene where samurai battled ninja).
Since then the focus has shifted towards the more familiar Pirates versus Ninjas conflict, and the day has served as a virtual counterpoint to International Talk Like a Pirate Day.
One of my very favorite Ninja movies:
You did not see me, I was never here!