Zamenhof Day, also called Esperanto Book Day, is celebrated on 15 December, the birthday of Esperanto creator L. L. Zamenhof. It is the most widely celebrated day in Esperanto culture. On this day, Esperantists hold information sessions and cultural gatherings to promote literature in Esperanto.

Esperanto is the world's most widely spoken constructed international auxiliary language. Created by L. L. Zamenhof in 1887, it is intended to be a universal second language for international communication, or "the international language". Zamenhof first described the language in Dr. Esperanto's International Language, which he published under the pseudonym Doktoro Esperanto. Early adopters of the language liked the name Esperanto and soon used it to describe his language. The word esperanto translates into English as "one who hopes".