Iran has a nice whether in Spring at Ordibehesht, except southern cities which are hot and sultry at that time. Most southern parts of the country have got warm and sultry weather which make them two-season.
In summer, climate differs in all around the country. Mazandaran's cities are warm and sultry while there mostly rains. Southern parts of the country are too hot and sultry to travel. North west of the country, Especially Ardabil, is cool and fine and appropriate for tourists destination. Other parts of the country have warm and dry weather which intensify as you go from north to south but you can find places in mountainous areas with fine and good weather.
Most cities in Iran have nice and fine weather in spring and autumn. Nowruz Holidays, the first weeks of the year is the most appropriate time for traveling.
The best time to travel to south and southern islands is autumn and winter. the weather then feels like spring. In north the weather is misty and cold when it rains over colorful nature. the tempting deserts of Iran calls you at these times when walking under weak sun and a little bit cooler weather is possible. although days in desert are not very cool but nights get cool enough. In Lut Desert, in Kerman or South Khorasan the temperature is above 30 degrees Celsius in days and falls down to -10 degrees at nights in winter.
Lastly, when ever you feel like traveling to Iran, nature serves you with a diversity of snowy mountains, sandy beaches, golden deserts and foggy jungles.