I am so tempted to drop this onto my fantasy shelf, because as I read this to my nephew it was clear that this sort of day would absolutely be a child's fantasy! His eyes lit up, and he was grinning like a fool from cover to cover.
The end papers were clever, too; a calendar in which every day is some form of "no day," until the final day of the month when it's Yes Day. We read each of the "no day" notations on the calendar, and when we finished my niece said, "Man. That is one bad month."