Almost every weekend there is a fair in one of the local towns. St. Marys boasts its Stonetown Heritage Festival and an Agricultural Fair, both in July. Mitchell has its Cornfest. For Wellesley, it's the Apple Butter and Cheese Festival. In Goderich, there's a Celtic Festival. Finally, we must mention Oktoberfest in Kitchener/Waterloo.
You can check out what's happening everywhere in Perth County at this site: http://www.visitperth.ca