accordion festivals

Every February there is a big accordion festival here in Vienna. http://www.derwilhelmshof.com/blog/en/international-accordion-festival-2013-in-vienna/ I regret to say that I've never been.

You might be interested in the book "Accordion Crimes" by Annie Proulx. I recommend it to all accordion lovers.

If you're ever in Brussels, the Museum of Musical Instruments has a great display of many different types of accordions. The free headset allows you to listen to substantial pieces of music made by several of them. One of the best, most enjoyable museum experiences I've ever had.

The annual Django Rheinhard swing jazz festival at Samois sur Seine, just outside Fontainebleu (itself just outside Paris), will involve accordions although not as a principal instrument. It's at the end of June. http://festivaldjangoreinhardt.com/
On another continent, you might be curious to read about the annual Carrefour mondial de l'accordéon in Montmagny, Quebec. It's held on the Sept. 1 weekend in a ski resort town, south side of the St. Lawrence River about an hour's drive northeast from Quebec City. Young and ambitious international players are balanced by traditional performers, at both free and paid concerts. The accordion was used for dance music in Quebec but rooted in fiddles and step-dancing rather than polkas. http://accordeonmontmagny.com/