Galicia has a reputation for rain, and Pontevedra earns it — the city gets around 1,600 mm of rainfall a year, mostly between October and March. Spring and early summer (May to June) are the most pleasant months: temperatures are mild, the old town is not yet packed with tourists from the rest of Spain, and the countryside around the Rías Baixas is green and vivid.
July and August are warm and busy. The city fills with domestic tourists, and the squares and bars are lively. Expect temperatures in the low to mid-20s Celsius and occasional heavy Atlantic showers even at the height of summer.
The most important local festival is the Festas de Pontevedra in late August, when the old town hosts concerts, markets, and processions. The Semana Grande in early September is another major event.
Avoid late November through January if you are sensitive to cold, damp weather. The old town in midwinter has a particular grey beauty, but it takes a certain cast of mind to appreciate it.