The best times to visit during the peak season are the bookends—May and June or October. During late spring and early summer the weather is sunny and generally quite pleasant for lots of walking around (and you will be doing lots of walking around). Same goes for late autumn, when the weather takes on a crisp quality that’s refreshing and well complemented by local pumpkin pie. July, August and most of September, on the other hand, are painfully hot and humid, and crowded besides.
You’re taking a bit of a gamble if you come at this time. If you're traveling with kids, there’s a good chance the most kid-friendly attractions in the Triangle, such as the amusement parks, will be closed and the weather may be gray, wet and nasty. On the other hand, you may catch some unseasonably beautiful days as either fall dies or spring begins. Either way, you’ll be dealing with a lot less crowds.