Knowing by and extremely long and boring theatrical performance the Top members of the family had to go trough, the celebration was instated by the Devils when the foundations of Hell administration where made.
The event consist of two Magic User sing each other Contracts (special organs located inside their chest, with the appearance of actual papers made of flesh) in the Black House, signed by a Devil, sometimes Chidaruma himself do this.
The partnership can be a mutual accord or a sorcerer can manipulate or even force another magic user to sign the contract, doing it in this way renders the affected unable to have a will of his own. This prompts many shady magicians to kidnap and sell powerful or rare sorcerers for this event.
The partnership last four years until the next blue night, and the contract inside the magic user becomes blank again.