Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Washington Dulles Intl Airport to Penang as you normally would.
Due to multiple factors, expect to potentially pay more when booking in December. For low rates, October can be a great choice for travel to Penang. Flights to Penang from Washington Dulles Intl Airport in October are about $1,187 on average during October, but can be found for as low as $674.