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 Adelaide as you normally would.
Due to multiple factors, expect to potentially pay more when booking in August. For low rates, November can be a great choice for travel to Adelaide. Flights to Adelaide from Washington Dulles Intl Airport in November are about $1,650 on average during November, but can be found for as low as $807.