October to April is the right time for Kish. Temperatures sit between 18°C and 28°C, the sea is clear, and the coastal path is a pleasure to walk or cycle. Iranian holiday periods — particularly Nowruz (Persian New Year, around March 20) and the summer holidays in July and August — bring large numbers of domestic tourists and push hotel prices up significantly.
Summer is genuinely difficult: temperatures regularly exceed 40°C with high humidity, making outdoor activities unpleasant before 7am and after 6pm. Avoid June through September unless you're primarily here for the air-conditioned malls and indoor attractions.