Eastercon 1, Whitcon, was held at the White Horse pub in London on May 15-16, 1948 (Whitsun weekend, hence the name.) The GoH was A. Bertram Chandler. It was run by Walter Gillings and John Newman.

It's ironic that Eastercon didn't start on Easter.

from Fancyclopedia 2 ca. 1959
The gathering (brainstormed by John Newman) that marked the postwar revival of congoing in England. It took place 15-16 May 1948, over Whitsuntide weekend, at the White Horse tavern in London. Nearly 60 fans attended, including Gillings, Carnell, ACC, Bill Temple, and Bertram Chandler. New Worlds was declared folded (ending the last surviving English prozine), and a cooperative company was planned to take it over.