In general, when booking your flight to Lisbon, November is the cheapest month on average.
From most locations, booking your flights to Lisbon around 45 days prior to your departure date finds the best deals.
In general, booking your departure to Lisbon for a Tuesday represents the best deal on airfare. Try to avoid a Friday departure to stay away from the highest prices.
Typically, cheap prices to Lisbon are most often found for midday flights. Cheap prices are less likely for morning flights.