We do not have enough data on this route to provide a reliable graph showing how many days before departure it is the best to book.
However, usually the closer it is to the departure, the more expensive the tickets are.
The most popular airlines on the route Lisbon - Helsinki: TAP Air Portugal (TP).
The cheapest flights you can book on the route Portela (LIS) - Vantaa (HEL).
Direct flights last about 4h 35min.
Direct flights available 3 days a week - Monday, Wednesday, Saturday.
Direct flights: TAP Air Portugal (TP).