Marvel held its own comic book convention, Marvelcon '75, in spring 1975, and promised a Marvelcon '76. At the 1975 event, Stan Lee used a Fantastic Four panel discussion to announce that Jack Kirby, the artist co-creator of most of Marvel's signature characters, was returning to Marvel after having left in 1970 to work for rival DC Comics. In October 1976, Marvel, which already licensed reprints in different countries, including the UK, created a superhero specifically for the British market. Captain Britain debuted exclusively in the UK, and later appeared in American comics.

What was Marvel's self-hosted comic book convention called?