No release date, platform or region has been announced yet. Source: Official announcement
The staff of the upcoming Madoka Magica film have requested fans to refrain from posting spoilers online until September 12. This request applies to all social media and video sharing platforms, ensuring that all viewers can enjoy the film without preemptive story details. The teaser scenes will be previewed during the revival screening of "Rebellion," which begins on August 14. The staff asked for this so that as many people as possible can enjoy the film.
Announced by madoka-magica.com
Official source: madoka-magica.com