I didn't attend any "events" but stuff is organised for people from mostly (all?) Internet based forums. There are informal meets and meets that involve a charge (and you get an event in exchange).
I believe (though I'm not 100% sure) that Mousefest is deliberately timed to be at the quietest time of year at WDW
ETA - I don't think this is a Disney endorsed event, it is the internet groups that do it all.
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