Orphan Jax Aubrey doesn't expect much on his thirteenth birthday, but when he discovers there's an extra day squeezed between Wednesday and Thursday whose origins are rooted in Arthurian legend, it's clear his life will never be the same.

"For years, the battles between Kin and Transitioner have been inching dangerously close to all-out war. Not the Kin may have just what they need to tip the scales in their favor: Addie Emrys, Evangeline's sister and one of the last remaining descendants of Merlin. If the Kin can get their hands on the Sword of Nuadu and the Stone of Fal, then Addie's help will allow them to break the Eighth Day spell, unleashing chaos upon the world. But Jax won't...