In general, when looking for the cheapest flight to Bilbao, try to choose a flight departing in January for the best deal.
Most flyers found that booking flight tickets 40 days in advance of their planned departure date is, on average, the best time to get cheap flights to Bilbao.
From most locations, the cheapest flights to Bilbao are found on a Saturday. Booking a Wednesday departure is usually the most expensive.
On average you can find the best deals on plane tickets to Bilbao when looking for flights with morning departure times.