14 December is the day when former 47 samurai of Banshu Ako Asano domain in current Ako, Hyogo prefecture raided Kozunosuke Kira's residence in Edo(current Tokyo) and killed Kozunosuke on 14 December 1703.
This raid happened due to the 47 samurai's master, head of Asano domain, Takuminokami Asano was forced to do harakiri by the Tokugawa shogunate, because Takuminokami tried to kill Kozukenosuke by his katana sword at one of the main corridor of the Edo castle, shogun's White house in the spring of 1701. Asano domain was forced to move to other place and the Ako castle was taken over by the Tokugawa shogunate.
Luckily Kozukenosuke was not killed and not punished by the Tokugawa shogunate. During Edo period, both sides will be punished for any fight or brawl. Therefore the samurai of Asano domain thought is not unfair and planned to raid Kozukenosuke's residence and kill him as the revenge.
At that time, any misuse of katana sword inside the Edo castle will be punished as harakiri and therefore Takuminokami's harakiri is no way to blame the shogunate. Also, it is not clear why Takuminokami tried to kill Kozukenosuke.
Anyway, the story of the revenge is called Chushingura and it is one of the famous story which was been played on TV by numerous actors for many time. However, I consider this story as a terror, killing high rank officer for uncertain reason.