The best time to visit is between April and May and again in October. These months offer mild weather around spring, the edge of summer, and fall. During April, May, and October there are also fewer crowds.
June, July, and September are very busy in Portugal because of an influx of cruise ships to Lisbon and around the Algarve. Summer has the best weather for Algarve beaches, but also sees the largest crowds.
There are different climates in the southern, central, and northern regions, with the Algarve being too cold for the beach outside of summer and at the beginning of fall.
Learn more: Best Time to Visit Portugal