Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Portland to Portimão as you normally would.
Due to multiple factors, expect to potentially pay more when booking in December. For low rates, March can be a great choice for travel to Portimão. Flights to Portimão from Portland in March are about $998 on average during March, but can be found for as low as $621.