What is an Eastercon?
The "Eastercon" is the British National Science Fiction convention and has been held annually since 1948 (in recent times over the Easter Bank Holiday weekend, though in earlier years it was held in May over the Whitsun Bank Holiday weekend). There were earlier national conventions going back to 1937 that some people include in the list.
It is for fans of Science Fiction, Fantasy and associated genres. It is a good general interest convention (rather than being focused on one TV show or just on written SF) and is a great way of trying out different things (gaming, media SF, filking, writing etc.) while spending a fun (long) weekend with like-minded people.
The average attendance is around 7-800 people including a number of authors, publishers, artists and others who work in the Science Fiction and Fantasy field. People travel from all over the UK, plus a number of fans come from Europe (including Ireland and The Netherlands) and further afield (there are always a few Americans, often a few Australians, and many others).
There are many specialist conventions held throughout the year for people with a particular interest in one detailed area of Science Fiction and Fantasy (Star Trek conventions, Dr.Who events, Tolkein Society events, Filk conventions etc.) but the Eastercon is a place where all the fandoms can come together.