The Napa Valley is good to visit year-round, but the experience changes sharply with the season. Spring (March–May) brings green hillsides, wildflowers, mustard in the vineyard rows, and mild weather — many visitors' favourite time. Summer (June–August) is hot, often over 35°C in the afternoon, but mornings and evenings are pleasant and the long days are good for outdoor tastings. Harvest (September–October) is the busiest and most expensive season — vineyards are working, tasting rooms are full, hotel prices peak, and many wineries hold special events. Winter (November–February) is the quietest time, with cool damp weather, frequent rain in December and January, and the best chance to actually talk to a winemaker over a tasting.
A few practical notes. Avoid driving from San Francisco to Napa on Friday afternoons between 2pm and 7pm — traffic on Highway 29 and the Silverado Trail can add an hour or more. Many wineries close one day a week, usually Tuesday. The last two weeks of December and the first two of January see a wave of seasonal closures.