The Universala Kongreso, or Universal Congress, is the first and largest annual Esperantist convention. It is organized by the UEA. The UK has been held every year since 1905, except during the World Wars.
The Congress is a major event for the Esperanto community; for many it is the only opportunity to speak Esperanto in an international environment. It spans eight days, usually around the last week of July. During that time there are concerts, banquets, plays, dances, discussions on various topics, and anything else the organizers dream up. Each Congress has a theme around which these things are centered; the one for 2001 (which I can translate better than the 2002 one) was "Culture of dialogues - a dialogue between cultures".
The location changes every year, and depends on invitations from local Esperanto groups. These require a lot of paperwork; among other things they must include official invitations from both national and local tourism authorities, an official invitation from the city, and a statement from state authorities that all registered participants will be permitted to enter the country.
Because most Esperanto groups are still based in Europe, the UK occurs there most often. However, in an effort to strengthen the movement worldwide, the UEA tries to hold the Congress outside of Europe at least once every four years.
Thanks to: http://www.uea.org/angla_p/kongresoj/