No release date, platform or region has been announced yet. Source: Official announcement
Screenings of the film BanG Dream! Ave Mujica prima aurora will open with "Ave Mujica Manners Lesson Theater," a short movie on theater etiquette shown before the main feature, the film’s official website announced.
The short is set in the world of Original! Bandori-chan, the series’ chibi spin-off, and is produced by AtoriE.
announced by the official Ave Mujica website
Official source: avemujica-movie.bang-dream.com