When to Go
It's best not to visit at the height of summer, when the city swelters in the heat, many smaller restaurants and shops close for family vacations, and lodging is expensive and crowded. Winters are generally mild and usually accompanied by bright blue skies, and there are plenty of bargains to be had at hotels. For optimum Lisbon weather, visit on either side of summer, in May or late September through October. The city's major festivals are in June, when the Santos Populares (popular saints) festivals see days of riotous celebration dedicated to Saints Anthony, John, and Peter.
You'll want to give yourself a day at least exploring the bairro of Alfama, climbing up to the Castelo de São Jorge (Saint George's castle) for an overview of the city; another in the monumental downtown area, Baixa, and in the neighboring fancy shopping district of Chiado, and perhaps also in the funkier shops of Bairro Alto. Another again could be spent in historic Belém, with its many museums and monuments. Note that many are closed Monday, and churches often close for a couple of hours at lunchtime. Leave time in your schedule to take the ferry across the river to Cacilhas, for excellent fish restaurants and a handful of hidden waterfront bars that offer superb views of Lisbon.