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 Witches of the Tower 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.