April 23rd is celebrated in many parts of the world as World Book Day (or The International Day of the Book or Copyright Day) ever since UNESCO (United Nations Educational, Scientific and Cultural Organization) so designated the day in 1995. The holiday originated with the Catalonian tradition for men to give roses to loved ones on St. George's Day (April 23) and for women to reciprocate with a book. As a result, half of Catalonia's annual book sales are made during this period.
UNESCO chose April 23rd as World Book Day partly because of the Catalonian festival but also because a number of well known authors were either born or died on this date, including Miguel de Cervantes, Vladimir Nabokov, and Shakespeare.
In Catalonia, April 23rd is a celebration of books and reading. Outside stalls and stands crop up for purchasing books at a discount, authors have book signings, and schools have book-related field days. In the UK, World Book Day is celebrated on March 6, so as not to clash with St. George's Day (April 23rd) since St. George is England's patron saint.
The U.S. does not celebrate World Book Day.