Lost Christmas is the event that caused mass chaos through Japan, and started the conflict of the series. It started on December 24th 2029.
Everything started when Mana Ouma found the meteorite. She was infected and decided to make her brother, Shu Ouma her Adam. The summer before Lost Christmas, Mana and Shu met and rescued Gai Tsutsugami, and befriended him. But Gai saw that Mana was infected by the Apocalypse Virus and was insane. At the same time, Mana and Shu's father got remarried and their new stepmother wanted them to move to Tokyo.
On December 24th, 2029 Gai called Shu to a church to tell him of Mana's insanity, but Mana got there first. She gave Gai, a gun, and told him to shoot the star at the top of a christmas tree. He did as he was told, but the shot recoiled and hit him back. Mana simply laughed and used his blood as lipstick. Shu eventually got to the church, to find Gai in a pool of his own blood and Mana. Mana told him to tie the knot with her, so their genes could repopulate the Earth. But Shu saw part of Mana's hair crystallize, causing him to freak out and call her a monster. Mana in sadness used her new powers to cause destruction in the Church. Mana, frighten and asked Shu for help, but he was too scared to help, causing her to unleash all her power. Causing Lost Christmas, but destroying herself, while Gai shielded Shu.
The event spread the Apocalypse Virus through out Japan. It was so destructive that Japan was nearly destroyed and the UN was forced to intervene, and created GHQ to control the infection. Eventually they found a cure to the disease, but it had to be taken regularly, to stop the disease. GHQ was left in control, but many Japanese disliked being other's control, and revolutionary groups like Undertakers were created to regain Japan's independence. Shuichiro Keido and Daath unwilling to let Mana stay dead, created Inori Yuzuriha to act as a vessel for the dead Mana to allow the fourth apocalypse to occur. Shu would forget these events, to ignore the pain and suffering, while Gai left to get stronger.