Are you planning a theremin fest?
One of the difficulties planners of past festivals have had is that the theremin community is spread out, and it can be very costly for people to travel long distances and inconvenient to travel with electronic equipment.
Europeans are fortunate because they are dealing with a much smaller geographical area, and it is easy for them to travel from one place to another. We in America forget that the distance between Los Angeles and New York is roughly the same as the distance between London and Baghdad!
If you are planning a theremin festival, what is your goal? Do you want to your event to be a high impact artistic happening or do you see it as a meet & greet, "open mike" social get-together?
Over the years I have attended many musical special interest group conventions and conferences and hosted one or two (including the Yamaha Festival) as a professional MC, and the tone of them has always been set by the virtuosity and expertise (or lack thereof) of the headliners and "star" participants. For instance, the 1997 Portland Theremin Festival (which I attended) featured the participation of Lydia Kavina, Robert Moog and avant-gardiste Eric Ross. Without them, the event would have had little to recommend it.
Olivia Mattis, the driving force behind Portland, had to cancel her second attempt at launching a theremin festival because of lack of interest.
Where would you hold your festival? How long will it last? These are important considerations because of the high cost of hotel accommodations, restaurants etc., especially in major metropolitan areas like Manhattan. Don't forget, the vast majority of theremin enthusiasts are "on a budget".
Don't expect to make a profit on your festival. You'll be lucky if you break even (unless you can persuade Justin Bieber to participate).