December 15 (Zamenhof Day, Zamenhofa Festo) is the birthday of L. L. Zamenhof, the initiator of Esperanto. Many Esperanto speakers will buy an extra Esperanto book around this time of year and there are also special Esperanto gatherings (parties) throughout the world to celebrate the occasion. It is the most widely celebrated day in Esperanto culture.
Some Esperanto-speakers, not wanting to over-celebrate the achievements of a single man, have suggested celebrating December 15th instead as Esperanto Literature Day. Thus they encourage Esperanto organizations which have gatherings on that day to add a book review or poetry reading to the program or to announce the publication of a new book. Or on an individual level, one can buy a book or start reading a new book or in some way celebrate Esperanto literature on that day.