Susan,
   There is an annual accordion festival in Montmagny, Quebec, on Labor Day weekend: Carrefour Mondial de l'accordeon (tr. accordion world crossroads) de Montmagny. I'm a fiddler myself, but the music is so closely linked. Several years ago, I was passing through Montmagny, and stopped at the accordian shop of Marcel Messervier ( I was familiar with some of his compositions for fiddle). When I approached him to autograph my Portland Collection tunebook, he and his son entertained me with an impromptu concert that was one of the high points of my Canada trip. This is how I learned of the festival, which I understand Marcel has been "instrumental" in organizing.
    I also would like to recommend a Mel Bay book, "Danse Ce Soir", fiddle and accordion music of Quebec, which has a CD available.
    Hope this is helpful. I'll probably be in New England next summer, and would like to meet some fiddlers. Any suggestions on festivals/gatherings in August?