Which Was the First SF Convention?

Two meetings vie for the title: The Oct 22, 1936 meeting in Philadelphia (about a dozen fans) and the Jan 3, 1937 meeting in Leeds (about twenty fans). Some feel that the 1936 Philadelphia meeting was a mere visit of NY fans to Philadelphia and that the true first convention was the 1937 Leeds meeting in the UK.

This seems like hairsplitting, because small affairs like the Philadelphia meeting have subsequently been called "conventions" without objection. Both of them clearly thought of themselves as conventions, but it is certainly true that the Leeds meeting was larger, longer-planned, and more organized than the Philadelphia gathering.

Perhaps it would be fairest to say that the first thing that could be called a convention was held in Philadelphia in 1936, while the first thing that must be called a convention was held in Leeds in 1937.

See also Early Conventions.