In general when looking for the cheapest flight to Lisbon, try to book your flight during the month of February for the best deal.
From most locations, booking your flights to Lisbon around 32 days prior to your departure date finds the best deals.
From most locations, the cheapest flights to Lisbon are found on a Wednesday. Booking a Saturday departure is usually the most expensive.
On average you can find the best deals on plane tickets to Lisbon when looking for flights with midday departure times.