Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Washington Reagan-National Airport to Canary Islands as you normally would.
Due to multiple factors, expect to potentially pay more when booking in December. For low rates, May can be a great choice for travel to Canary Islands. Flights to Canary Islands from Washington Reagan-National Airport in May are about $997 on average during May, but can be found for as low as $431.