This year's National Charter School Conference will be June 21-24 in Washington, DC. The National Alliance for Public Charter Schools puts together the program.
I was at the first National Charter School conference held in Washington back in 1997. There were about 250 people who attended. Now about 2,000 attend each year. Further, there are now over 4,000 charter schools operating in the nation. California, Arizona and Florida are the states with the most charter schools; Colorado typially ranks about sixth on the list.