Light, amusing, properly, sentimental, "Thirty Day Princess" would be pleasant entertainment in any season, in spring it is nothing short of delightful. Making the usual concession to conviction it starts in the musical comedy kingdom of Taronia, which, despite the idy-llie happiness of its people, is sadly in need of these nice, fat, fifty-nine cent dollars. Santa Claus, in the person of an American banker decides to float a loan for Taronia. In order to impress the American people with the soundness of such an investment, he brings along the beautiful Princess Catterina (Sylvia Sidney). No sooner has the lovely lady put her foot upon American soil than she is smitten with the beauty of the land and a bad case of mumps. Rising to the emergency the banker finds a perfect double in a minor stock actress, also played by Miss Sidney. The serenity of her triumph is disturbed when a wealthy newspaper owner (Cary Grant) determines to protect the public against the machinations of the banker and the beautiful lady. His order is considerably dampened when he meets the young actress-princess and, as ever, Cupids aim is deadly. Simply, smoothly, and directly the story moves along to its happy ending. The simplicity of her role and her unusual beauty make it unnecessary for Miss Sidney to extend her histrionic talents which she has amply proven on other occasions. Cary Grant is tall, dark and handsome, and sufficiently capable even if he does sound as if his mouth were full.