In general, when looking for the cheapest flight to Lisbon, try to choose a flight departing in February for the best deal.
On average, from most locations, when planning your trip to Lisbon, try to book your flight in the region of 60 days in advance for the cheapest prices.
In general, booking your departure to Lisbon for a Wednesday represents the best deal on airfare. Try to avoid a Saturday departure to stay away from the highest prices.
Typically, cheap prices to Lisbon are most often found for morning flights. Cheap prices are less likely for evening flights.