Because it's on the Gulf Stream, Iceland is much warmer than most people think. The best time to visit is from mid June to mid September, when days average in the 50s F/10-15 C and nights are generally in the 40s F/5-9 C. From mid May to mid June and from mid September to mid October, it's still fairly nice, but with slightly cooler temperatures. The winters are cool and very windy, but mild, with days in the 30s-40s F/0-10 C and nights in the 20s F/-6 to 0 C.