Hooray for Diffendoofer Day! (ISBN 0-679-89008-4) is a children's book credited to Dr. Seuss "with some help by Jack Prelutsky and Lane Smith". The book is based on verses and sketches created by Seuss before his death in 1991, and was expanded to book length and completed by writer Prelutsky and illustrator Smith for publication in 1998.

The story surrounds a school that is well-liked by its students notably because of it's many eccentric teachers. However, the students must make a good grade on a standardized test or else they will be sent to an adjacent school, which requires uniforms to be worn and is incredibly dull.