The summer tourist season is from late May to early September. During the first half of this period the sun stays above the horizon for almost 24 hours and the interplay of light and shadows on mountains, lava fields and glaciers yield an ever changing landscape.
However, even during the middle of summer the sky is frequently cloudy or overcast and the sunshine does not warm the air much. Hence, during daytime, the air is usually cool ("refreshing" is the local euphemism) and cold during nighttime.
The winter season is the abode of long nights, and severe winter storms. However, the silence of the frozen expanse and the dance of the Aurora Borealis on a clear night sky draws an increasing number of tourists.
During summertime tourists should bring a windbreaker, rainwear, a thick pullover (wool or fleece) and sturdy walking shoes. Travelers who are camping or heading into the interior will need warm underwear and socks, rubber boots and a warm sleeping bag.
During wintertime tourist should bring warm clothing, warm coat, mittens etc. Iceland has many swimming pools, usually with geothermally heated water. Hence, in either season a visitor should bring a swim suit.