Hotarubi no Tomoru Koro ni (蛍火の灯る頃に,When the Fireflies Glow) is a manga written by Ryukishi07 and illustrated by Nokuto Koike. It was serialized in Gekkan Action from March 25, 2016 to February 24, 2018 and was later collected into four volumes.
Chapter 0 of the manga was published on the December 25, 2015 issue of Gekkan Action; while it wasn't included in the book volumes, it is available for free on the official site.