Due to multiple factors, expect to potentially pay more when booking in December. For low rates, August can be a great choice for travel to Key West. Flights to Key West from Dubuque in August are about $452 on average during August, but can be found for as low as $217.
Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Dubuque to Key West as you normally would.