I was thinking about how Star Trek had become such a cultural obsession for a certain section of the population and how conventions for it became such a stable industry. Was Star Trek the first TV show that inspired these kinds of conventions? Were there any pre-television celebrities that had similar phenomena? Like the cast of an Opera meeting fans in an organized fashion 20 years after their theater run ended.
What constitutes the first "con" depends on how you define a convention. There are two possible first conventions: one between New York and Philladelphia Sci-Fi fans in late 1936, and another in Leeds, UK in January 1937. The former centered around Forrest J Ackerman's "The Boys' Scientifiction Club" with futurist Frederik Pohl acting as guest speaker - "convention" being a loose term since this was held at Ackerman's house. The Leeds convention featured writers like Arthur C. Clarke. Both were mostly meetings to discuss fandom-related issues, like fan club organization.
The World Science Fiction Convention, the first convention of what is now "Worldcon," took place in 1939. It was held in New York to coincide with the New York World's Fair. It was more recognizable as a modern convention with open attendance guest speakers. Naturally, this was followed immediately with the first protest and counterconvention. :)
No, really - there was a decision to exclude "futurists," so they created their own convention to coincide with WorldCon. Frederik Pohl, one of the excluded authors, wrote about the counterconvention in his autobiography "The Way The Future Was." He claimed the issue came down to the futurist writers acting like they were better than the more run-of-the-mill sci-fi authors.
As for media-specific conventions, it depends on how you would define a 'con, but the first one was definitely Star Trek-related. There are claims that a 1969 meeting of fans at a Newark Public Library, an event inspired by "Spockanalia," the first Star Trek fanzine, was the first Star Trek convention. However, like the first sci fi conventions, this was just a meeting of fans to discuss the show. The first modern franchise 'con was probably a 1972 Star Trek convention held in New York. It featured guests involved in the show's production, including Gene Roddenbury who showed the now infamous "blooper reel."
Sources: Criterion Collection's article on the auction of Ackerman's estate
Rob Hansen's "Fan Stuff" website is a great resource for this information. This page has photos from the first British convention as well as scans of articles written about it at the time.
The Long List Committee does research into fandom history and has been working on the "Long List of World Science Fiction Conventions" to create an accurate record of the long-running Worldcon's history.
Star Trek's own website claims the New York convention as being the first; they published [this article] (http://www.startrek.com/article/celebrating-40-years-since-treks-1st-convention) on the event's 40th anniversary with recollections by two of the attendees.
1978, Pohl, Frederik, The Way the Future Was