1st World Science Fiction Convention
The 1st World Science Fiction Convention (Worldcon) was held on 2–4 July 1939 in the Caravan Hall in New York City, New York, in conjunction with the New York World's Fair, which was themed as "The World of Tomorrow". It was later retroactively named "NyCon I" by Forrest J Ackerman.
Source: Wikipedia — 1st World Science Fiction Convention (CC BY-SA 4.0)