The central valley has a Mediterranean climate — dry, hot summers and cool, wet winters. The best months are October to December and March to April, when the days are warm without being punishing and the vineyards either flower or harvest. Summer (January and February) is hot and dusty; locals flee to the coast.
If you are interested in chilean rodeo, the National Championship is in late March or early April — book accommodation well in advance. Grape harvest in the Cachapoal Valley is from late February to April, when vineyards welcome visitors for tastings and lunches.
Winter (June to August) brings snow at altitude — sometimes enough to close the road to Sewell — but the valley itself is grey and damp rather than properly cold.