Spring — April and May in particular — is the best time to visit Kashan, and for one reason above all others: the rose harvest. The damask roses around Qamsar, a small town about 20 kilometres north of Kashan, bloom in late April and early May, and the distilleries run more or less continuously through the harvest period. You can visit the rosewater factories, buy fresh rosewater directly from the source, and smell the flowers drying in the sun outside the distillery doors. It is one of the more unusual seasonal experiences in Iran.
Outside the rose season, spring and autumn are both pleasant — temperatures are mild, the light is good, and the gardens (especially Fin Garden) are at their best. March brings Nowruz, the Persian New Year, which means Iranian domestic tourism peaks; the merchant houses and garden are busy but the festival atmosphere is worth seeing.
Summer is hot, often exceeding 40°C, and Kashan's position at the desert edge amplifies the heat. The windtowers in the merchant houses do their job, but exploring the city on foot between noon and 4pm in July and August is uncomfortable. Many Iranian visitors come in summer specifically for the architecture, which they prefer to experience in near-solitude. Winter is cold but dry and manageable; the city sees very few foreign tourists from November to February, which has its own appeal.
Ramadan timings shift year by year but are worth checking if you are planning around eating out — many restaurants reduce hours or close during daylight.