You can compare the weather in Isfahan to the Mediterranean European states. Isfahan has a mildly hot, dry summer from around May to October, and a cool winter with lots of snow between December and February.
If you are a winter sports enthusiast, visit any time from November to March – skiing, skating, and hot cups of local tea await you.
If you are a non Muslim, plan your trip in accordance with the holy month of Ramadan – Muslims fast during the day, and so restaurants and eateries around the city will be closed from sunrise till after dusk.
In general, the best time of the year to experience the city is during spring (March to May) and autumn (September to November). The weather is pleasant, neither scorching hot nor bitter cold, and makes for perfect outdoors sightseeing and adventuring.