Last year I missed Plaisance ---our flight into New orleans on Aug 31 was luckily cancelled!- (and we didn't become tourist-victims of katrina)... but I agree it is a beautiful day-long festival, and on the Monday following is the annual labor Day Boozoo fest in Lake Charles, a great lineup of music, food and family-oriented fun. Big Hat Fields has his big free bbq on the Sunday in between if I remember correctly.
But beware- it can be BLAZING HOT down there that time of year... often more than a Northerner can handle,
But I have great memories of sitting outside in 100+ degree heat with not a breeze anywhere, and humidity around 99%- sweat dripping from every pore while gnawing on a gigantic fried turkey wing (size of a baseball bat) and the grease drippings merging with the sweat.