Thinner crowds and balmy weather makes mid-April through May and September through October the best time to visit Lisbon. Late spring means orange blossoms all around. Plus, who can resist a glass (or a bottle?) during the fall wine season? These months have the lowest prices on hotels and flights as well. You can even put your finger on the beach season.
Times between June and August bring in skyrocketing rates and chords of tourists to the city and beaches. Fortunately, summer in Lisbon is almost rainless, which is handy for several festivals like Super Bock Super Rock or Sumol Summer Fest.
Even though temperatures significantly drop from November to February, the winter in Lisbon is relatively warm. Sure enough, you'll need rain gear for your promenades, but that's nothing compared to the expensive rates you’ll avoid.