April 25th marks the 50th anniversary of the Revolucao dos Cravos (Carnation Revolution), also known as "Dia da Liberdade" (Freedom Day) or simply "25 de Abril" (April 25), so a lot of partying is to be expected throughout the city, from the night of the 24th to the entire day of the 25th and beyond.
Among the many, many cultural events like art exhibits, concerts, movie screening and plays, there is a huge parade at Avenida da Liberdade ("Liberty Avenue") starting at around 15:00 (3pm).
On the night of the 24th, a little further south in Praça da Liberdade ("Liberty Square") in Almeda (~1h train ride or 25 minutes by car), celebrations will include a public concert with artists Dino DâSantiago, Batukeiras &Madame X (you may know them from a Madonna song), Tristany, Orquestra Geração and Virgul, starting at 22.00 (10pm). At midnight, there will be fireworks.
On the 25th, streets will be filled with carnation flowers, and many people will wear them on their chests. Please be aware that several streets, restaurants, stores and venues may not open on the 25th due to the national holiday celebrations.