Nix excels at world-building, which, as usual, is showcased in Saturday. Sorceress Saturday's realm is beautifully worked, with new classes of Denizen's, new personalities and gorgeously depicted scenery.
Saturday is also the most clear moral play of the books discussing the motivations and trustworthiness of the Will, the Old One, and Arthur himself as he progressively becomes less human.
However, I agree with other reviewers -- this reads like a work unfinished. Every other book contained gaining both the Will and the Key of that day, except Saturday. There is not really a clear contextual reason for the book to end -- it is neither a conclusion, nor a major cliffhanger, simply the book concludes.