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 Reykjavik. Flights to Reykjavik from Redmond in October are about $878 on average during October, but can be found for as low as $604.
Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Redmond to Reykjavik as you normally would.