Brave Witches: Petersburg Daisenryaku (Sub)
Watch Brave Witches: Petersburg Daisenryaku full movie online English Sub
Synopsis: The episode takes place between seventh and eighth episode and will be screened in theaters on May 13, 2017. The episode will also be bundled with the seventh Blu-ray and DVD volume of the anime, releasing on August 25, 2017.
More info: Myanimelist, Wikipedia.