Ålesund is a year-round destination but the summer months (June–August) offer the best weather and the longest days. In summer it barely gets dark — useful for exploring and for catching the light on the Art Nouveau facades. This is also peak cruise season, so the centre can be crowded on days when several large ships are in port simultaneously. Mornings and evenings are quieter.
Spring (May–June) is excellent: the light is good, the mountains still have snow above the treeline, and the crowds haven't arrived. Autumn extends the season with changing colours and fewer tourists.
Winter is dark, cold, and occasionally dramatic — the facades look particularly beautiful in snow, and you'll have the streets largely to yourself. Weather can be rough, especially in January and February, when storms off the Atlantic are not uncommon.
For fjord excursions — particularly Geirangerfjord — the summer window of late May to September is the only practical time. The Geiranger ferry road closes in winter.