Of course, the high season of tourism in Lisbon is June-August (as in the rest of Europe). Though at these dates the city gets very hot with temperatures reach above 30 C. the crowds at these months are also quite overwhelming.
Weatherwise, it is recommended to visit the city either March-May or September-October. At these times, the temperature is very pleasant, 20+- C, and the crowds aren’t so large.
November-February is a bit cold (of course not in comparing to north Europe), and there is quite a bit of rainy days. If you are looking for a great price, these are the months to travel.