Vik is accessible year-round but the experience changes significantly by season. Summer (June to August) brings long daylight hours, puffins on the cliffs, and the best weather, but also the largest crowds — the car park at Reynisfjara can be overwhelmingly busy on summer weekends. The Ring Road is generally clear and the roads to Dyrhólaey are open.
Winter (November to February) brings short days, frequently dramatic storms off the North Atlantic, and the possibility of seeing the northern lights on clear nights. The black beach in winter light, with rough surf and low cloud, is at its most atmospheric. Ice cave tours in the Mýrdalsjökull glacier operate primarily in winter, when the ice is stable enough to enter safely.
Shoulder seasons (September-October and March-May) offer a reasonable balance: fewer people than summer, more daylight than winter, and the possibility of both northern lights and relatively good weather. Puffins arrive in April and depart in August.
Whatever the season, dress for cold and wet. The south coast of Iceland is one of the rainiest and windiest parts of the country, and the microclimate around the glaciers can be unpredictable.