A full description of Eastercons is can be found at Wikipedia, including a list of all previous Eastercons with location, committee and guests.
Briefly, Eastercon is the common name for the British National Science Fiction Convention. From 1948 until the 1960s, the convention was held over the three-day Whitsun bank holiday at the end of May. Since then it has been held over the four-day Easter holiday weekend. The pre-1960s conventions are generally considered to have been "Eastercons" even though they were not held over Easter.
There is an Eastercon website at Eastercon.org but that is a work in progress.
Current and Recent Eastercons and Bids
Eastercon is a Trademark.