I am not sure when the first blueberry festival began. That would definitely cause a controversy. But, today there are 29 festivals on the USHBC website: (Surely, there are more...)
Blueberry Festivals begin in the spring in the South and end in the Pacific Northwest in the Fall. Check out the website for exact times and the dates change year to year.
What goes on at blueberry festivals:
Blueberry tasting, pie contests, concerts and all sorts of action.