1995.12.15 is a document in the White Day: A Labyrinth Named School (Remake).
It is Kim Ji-won's Diary.
A voice hidden among the kids...
Seo-yeon Kang asked me to find her voice for her since she wouldn't be here after her graduation...
학생들 사이에 숨어있는 목소리...
강서연은 졸업하고 없을 자신을 대신해 목소리를 찾아 달라 부탁했다...
- The name Seo-yeon Kang mentioned in this document is now known as the character appeared in White Day 2: Swan Song.
And the voice mentioned in this document is also believed to be related to the mysterious song mentioned in Synopsis of White Day 2: Swan Song.