Viña's high season runs from December to early March, when Santiago empties out to the coast and the city is at its liveliest. Summer days are warm rather than hot — typically 22–25°C / 72–77°F — and the Pacific stays cold year round thanks to the Humboldt Current. Out of season, autumn (April and early May) is pleasant and much quieter; winter is wet, grey, and best avoided unless you have a specific reason to come.
The single fixed point in the calendar is the Festival Internacional de la Canción in the third week of February. The town packs out, hotel prices climb, and every café has the live broadcast on. If you want crowds and atmosphere, this is the week; if you want a calm seaside holiday, time your visit for before or after.