Holy Day of the Calamitous Birth

Holy Day of the Calamitous Birth or The Disastrous Holy Day (災厄降誕のホーリー・デイ Saiyaku Koutan no Holy Day?) is a short story originally published in the Chaos;Head & Steins;Gate Science Adventure Series Maniacs (カオスヘッド＆シュタインズ・ゲート 科学アドベンチャーシリーズ マニアックス) information book, ISBN 978-4798601465. The story takes place after the true end of the visual novel.

In December in Akihabara, Rintaro Okabe is suffering from PTSD due to the events of the summer when he receives a letter from Kurisu Makise on December 14th, his birthday. However, things start to go wrong when Nae Tennouji suddenly goes missing on Christmas.