Even though weather-wise this is probably the worst time to visit D.C. – summers swelter with temperatures in the upper 80s – these months see the most traffic, probably because school is out and families are on vacation.
Off Season: December to January
D.C. is pretty popular year-round, but visitor numbers (and hotel rates) tend to drop in winter due to the freezing temperatures in the 30s.
Sweet Spot: March to May; September to November
This is generally a good time for sight-seeing – leisure travelers are few and temperatures average 55°F. Still, business and political travel shifts into high gear, pushing up hotel rates. March’s Cherry Blossom Festival also brings crowds.