It started with Noc Muzeów or The Long Night of Museums, on the 17/18 May. A translation of Polish and Icelandic poems, the night in the public library, and many other events related to book on this night.
The next event was Warsaw Book Fair, 22-25 May, on the National Stadium in Warsaw. What captured my heart this year was the comic section, as you can read in the linked post.
June is the time of the Pride Parade in Warsaw (called in Polish Parada Równości which means 'Equality Parade'). Since 30th May Equality Festival has started, a time for many LGBT-related events. It shouldn't be surprising there is also a small fair of LBGT-related books (9-15 June) In one of the caffe&bookstore place called MiTo. Nothing big, but still nice possibility to get some books in Polish and English for lower prices. The Parade itself is on 14th June.
Big Book Festival is a two days (14-15 June) festival in various places of Warsaw, and even beyond - in Podkowa Leśna, where is a museum of this year's big star writer - Jarosław Iwaszkiewicz. Many of its events are held in both Polish and English, which isn't common in Poland.
Have fun, I will on the Big Book festival where I volunteer.