DC is hot and muggy in the summer, with temperatures above 90 pretty common. The hotel is heavily air-conditioned. A temperature difference of 25 degrees Fahrenheit between inside and outside is not unusual. Cardigans, shawls, or hoodies may help you adjust as you move about the hotel or go out to restaurants, etc.
Dress for comfort. Attendees tend to favor casual dress, with a lot of t-shirts and jeans. Please do not wear a tiara, as that is the Con Comm's 'uniform'.