Natsu Ga Owaru Made Natsu No Owari The Animation Exclusive New! Page

"Summer doesn't end on a calendar date. It ends the moment you stop chasing the sun. This exclusive episode is that moment—the one between the last sparkler and the first school bell. We animated it not to conclude the story, but to let you hear the silence after the song."

That is the only official release.

Exclusive Look: Natsu ga Owaru made Natsu no Owari, A Poignant Coming-of-Age Anime natsu ga owaru made natsu no owari the animation exclusive