Faro and the rest of the Algarve are destinations that can be visited at any time of the year. The summer, high season from June to September sees crowded beaches and many families taking advantage of the weather, and local facilities to enjoy a great holiday, while during the winter, with the temperature still up around 20C, Portugal is a more relaxed place to get a bit of winter sun and very popular as a long stay destination for many would be ex-pats.
School holidays are always busy in the premier resort areas, and it is vitally important that you book a hotel in advance; otherwise you could end up sleeping on the beach.