more info here http://vndb.org/v11081

romanji: Yume ka Utsutsu ka Matryoshka

Description
Mamoru lives on the island of Shinonoshima, enjoying every moment with his osananajimi Miyako and close friends Isuka, Shiori, Nagi, Aoi and Kazushige. Their time together is irreplaceable to him and he believed that it would continue forever. However, he did not know he was about to fall into the abyss of despair.
He fell into a deep sleep due to a certain incident, and when he woke up, no one else remained except Miyako. Not wanting him to remain in sorrow, she told him about a ‘tradition’ passed down through generations. On the 100th night after the loss of their precious ones, if they are able to gather memories deeply connected with them and offer them to the island’s protective god Wadatsumi, then it might bring their lost ones back. They prepared for the ceremony while clinging on to the belief that it will work. [from Micchi's blog]