Travelling from most locations, it is best to book your flight to Bilbao during the month of May for the best deal, although there are other times of the year which offer great deals as well.
From most locations, the cheapest day to fly to Bilbao is Wednesday. Most users found that departing on a Monday was most expensive.
Typically, cheap prices to Bilbao are most often found for morning flights. Cheap prices are less likely for evening flights.