Iran has a very different climates in different regions in a certain season. For example, when it is summer, the weather is very hot in the southern cities of Iran like Bandar-Abbas and Bushehr, however, the Tabriz weather in the north of Iran is very favorable. Since I have lived in some Iran cities in the last days, I can describe some weathers and good seasons for travelling of some important Iran cities that I was in them.
Shiraz: Its weather is almost favorable in all seasons, but, I recommend to travel to Shiraz in the spring (I mean March, April, May and June) to feel the best spring weather of Iran. In summer, it is a little warm (about 35C)and in winter, it is a little cool (about 10C). I can ensure you that you enjoy its climates in all days of a year!
Tehran: It is similar to Shiraz, but, because of pollution in the air, it seems very hot in summer and I do not recommend to travel to Tehran in summer. There is nothing special about Tehran weather in spring or autumn. It is cold in winter (about 0C) and Hot in summer (about 45C).
Rasht: It contains a tropic climate, because of Caspian Sea. Very humid but very favorable in all seasons. The variation of temperature is little in different seasons (between 10C in winter to 35C in summer).
Kermanshah: It is located in a mountaneous region in the west. Good climate, but very cold in winter days (less than 5C). Its climate is very good in summer and spring.
Semnan: It is very hot in summer, but its winter is favorable. There are hot winds in the summer days in this city and make its weather very dry and very hot, just like a hair dryer in the face! But, its climate is favorable in autumn and spring.