The dry season runs from May through October and is the standard answer for when to visit La Paz. Skies are clear, the altiplano light is extraordinary — harsh and shadowless at midday, golden at dusk — and rainfall is rare. Nights get cold year-round at 3,600 metres, but in July and August overnight temperatures in the city can drop to near freezing. Bring layers regardless of when you come. The rainy season (November through March) brings afternoon downpours but mornings are often clear, and the surrounding hills turn green in a way they don't in the dry months. Tiwanaku and the salt flats look different in each season: Tiwanaku is more atmospheric in the wet, the Salar is stunning when a thin layer of water turns it into a mirror.
La Paz has two festivals worth timing a visit around. Alasitas, held on 24 January, centres on Ekeko — the Aymara god of abundance — and involves buying miniature versions of things you want in your life (a house, a car, a visa, a diploma) and having them blessed. The fairground around Plaza Alasitas is dense and specific and unlike anything else. Gran Poder, held in May or June depending on the year, is La Paz's largest festival: tens of thousands of dancers in elaborate costumes wind through the streets in a procession that takes most of a day to pass. Altitude affects everyone arriving from lower elevations. Allow at least two days of gentle acclimatisation before attempting significant physical activity, and drink coca tea rather than coffee on arrival.