From www.ibby.org :
Since 1967, on or around Hans Christian Andersen's birthday, 2 April, International Children's Book Day (ICBD) is celebrated to inspire a love of reading and to call attention to children's books.
Each year a different National Section of IBBY has the opportunity to be the international sponsor of ICBD. It decides upon a theme and invites a prominent author from the host country to write a message to the children of the world and a well-known illustrator to design a poster. These materials are used in different ways to promote books and reading. Many IBBY Sections promote ICBD through the media and organize activities in schools and public libraries. Often ICBD is linked to celebrations around children's books and other special events that may include encounters with authors and illustrators, writing competitions or announcements of book awards.
A catalogue of posters and messages that have appeared between 1967 and 2002 is available from the Secretariat; the price for bulk orders is available upon request.
I think I'll read Thumbelina to celebrate!