Forty Seven Samurai
December 14 commemorates the same day in 1702 when forty-seven ronin (浪人, masterless samurai) avenged the death of their master by beheading the court official who had forced their master to commit suicide. The ronin were later forced to commit ritual suicide for their involvement in the murder. Not a particularly happy story, but one that is known by all Japanese people, so a good cultural point to read up on. See the article on the forty-seven ronin in Wikipedia.