Lima sits in the Humboldt current's rain shadow — a fog desert on the Pacific coast. The result is a city with no real rain but heavy grey cloud cover for half the year. The summer (December to April) is the warm, sunny half: 25–28°C, brilliant skies, and busy beaches. The winter (May to October) is cool, damp, and overcast — temperatures rarely fall below 14°C, but the garúa (sea mist) blankets the city for weeks and you may not see the sun.
For a city visit, summer is more pleasant; for the food scene and museums (which work equally well in fog), winter is fine. The main religious festival is the Señor de los Milagros procession in October — central Lima fills with purple-robed pilgrims for several days. Independence Day on 28 July brings a long holiday weekend and patriotic parades. Avoid the days around New Year's Eve if you don't like crowds on Miraflores beaches.