Early Conventions
The first ten years of SF conventions:
Date Convention Attendance
1936 October 22 First Convention in Philadelphia later called the First Eastern. 10-20
1937 January 3 Other First Convention in Leeds, UK ~20
February 21 Second Eastern in New York 30-40
October 30 Third Eastern Science Fiction Convention in Philadelphia
1938 April 10 Second British Convention
May 29 Newark Convention aka Fourth Eastern aka First National >100
October 17 Philadelphia Conference > 2 dozen
1939 Third British Convention in London
July 4 NyCon the first Worldcon 150-200
Fall Philcon 1939
OSA Pow-Wow Oklahoma state convention
1940 April Third Chicago Conference 4
Sept 1-2 Chicon 128
November 10 Phillycon 1940
1941 late Feb Boskone I ~ 25
July 4-6 Denvention ~100
November 16 1941 Michiconference
1942 Feb 22 Boskone II 25
September 27 1942 Michiconference ~25
November 29 Hastings S-F Conference v. small, possibly a drinking party
Pacificon Jr.
1943 Feb 28 Boskone III 14
Spring Midgicon in Chicago 10?
Spring Live Oak Conference 3
Easter Midvention
May 16 The first Staplecon in San Francisco ~12
Summer MeCon in Rockland, ME 3
mid Mainecon Jr somewhere in Maine 2
"mid" Schenectacon 2-4
October 30 1943 Michiconference 22
November 21 The second Staplecon in San Francisco
"around the end" Fancon
1944 Easter Eastercon 1944
May 28 The last Staplecon in San Francisco 9
June 17-19 Michicon 4 23
October 7-9 Little Chicon "small"
1945 February 3-4 Boskone IV 5
July 7-8 1945 Michiconference 15-20
September 2 First New England SF Conference AKA Boskone V 9
1946 March 3 FPWESFC
August 30-September 1 Pacificon I 130
October 27 Phillycon 1946

See the Fancyclopedia 2 article on Conventions.