Walpurgis Night

Walpurgis Night (魔女たち(ヴァルプルギス) の夜) is a great "event" referring to the war between Tower Witches and Workshop Witches because of the white princess sealed inside Honoka Takamiya. It was first mention on Chapter 3.

Trivia

 * "Walpurgis Night" might referenced from the same-named event, which held in North Europe for celebrating the season of Spring.