Esperanto 26 is Esperanto's birthday, i.e. the date the first book ("Unua Libro") was published in 1887. The British Library commemorates this occasion this year:
Through the world a mighty voice is ringing… by Olga Kerziouk, British Library, 25 July 2014
Note links to other Esperanto pages on the British Library site. The British Library contains holdings on aspects of Esperanto, including the Universal Esperanto Congresses featured here, among them the ill-fated 1914 conference (see postcard above) nullified by the outbreak of World War I, and the British contribution to the Esperanto movement.