Thirteen year old Jax is living with a guardian who doesn't truly take care of him, so when he wakes up to a world with no people in it, he is frightened, and, remembering movies he has seen, assumes it's a zombie apocalypse. He robs a store and hides the stash, but later finds out from eighteen-year-old guardian, Riley Pendare, that he is one of the few who can visit a special extra eighth day between Wednesday and Thursday. Jax and Riley are part of a group called Transitioners, who are able to live in all eight days. There are others who never know about the 8th day, and still others, including Evangeline, the ghostly girl hiding in the house next door to Riley, who exist only on this special day.
The reason Evangeline is hiding goes back into the far past, into another story of King Arthur and the legends around Merlin and the Round Table. Jax is pulled in, and has to deal with regular human evil as well as the stunning revelations of the changes to his world, and of his family's past. How he handles it is part of the wonder of the story, as Jax struggles to help his new friend while being drawn into a new apocalypse and a struggle to destroy the human world.
A definite reread, and heartily recommended for all age groups.