Hooray for Diffendoofer Day! is a children's book credited to Dr. Seuss "with some help from 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 poet Prelutsky and illustrator Smith for publication in 1998.

Theodor Seuss Geisel was an American children's author and cartoonist. He is known for his work writing and illustrating more than 60 books under the pen name Dr. Seuss. His work includes many of the most popular children's books of all time, selling over 600 million copies and being translated into more than 20 languages by the time of his death.