What is "eastercon"

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. It has taken place over the four-day Easter holiday weekend ever since then. The pre-1960s conventions are generally considered to have been "Eastercons" even though they were not held over Easter.