Ronald and Gillian Saunders had a very happy life on Manhattan's Upper West Side, where life was "usually pleasant, and sometimes pleasantly unusual." The "unusual" was created by their magical cat Merlin, who introduced them to the astonishing (and hidden) thirteenth floor of their building, and who made sure that the thirteenth of every month included a magical adventure.
Ronnie and Jill had many astonishing experiences, from meeting Oscar, the mouse-proprietor of the "A Complete Line of Magic" stall, to their friendship with Mrs. Wallaby-Jones, a kangaroo-escapee from the Central Park Zoo. An unexpected blizzard that provides an opportunity for sledding in Central Park, a fortuitous fog that transforms the 125th Street Ferry into a trip through time, the purchase of a Genuine Swiss Superior Cuckoo Clock that allows Ronnie to slip inside and, with the help of miniature inventor Homer P. Maxurbanipul, gain his "mechanical aptitude" - anything could happen on the 13th!
The 13th is Magic was an enjoyable piece of fluff - the sort of episodic-magical-adventure-fantasy that was quite popular, before epic quest-fantasy came to dominate the genre. I loved Merlin, enjoyed many of the adventures, and agreed with Jacob (the elevator-man) that "Nobody ever owns a cat." I could have done without the unfortunate incident involving the 157 Indians who all looked alike and only knew how to say "Howgh," but other than that, this was a pleasant enough read.