Summer (June to August) is the easiest time to visit: the days are long, the ferry runs twice daily, and the Snæfellsnes peninsula is at its most accessible. The Baldur ferry operates a reduced winter schedule, but Stykkishólmur is a year-round community and accessible in all seasons by car.
Winter has its own appeal — the northern lights are visible on dark, clear nights, and the town sees far fewer visitors. The Glacier National Park at Snæfellsnes is dramatic in winter but requires more care on the roads. The main thing to check is the ferry schedule if you're crossing to the Westfjords.