Why does the midnight sun have the name midnight sun?

Answer 1

During and before WWII the flag of Japan had the rising sun on it. As Japan was the eastern most country of Asia many ancient Asians believed that Japan was the home of the rising sun, as the sun rose out of the east. After WWII the flag was changed to a simple sun. The title of the novel suggests that Japan is sinking into a decadent night as the customs and traditions of Japan as being corrupted and lost to external ways.