As we mentioned in our Q & A on this subject, you can travel to Iran at any time of year. But we must admit that for most people the spring and fall months are the best time to visit Iran. The summer is very, very hot in the South of Iran and the South is where many of the most important places of interest to tourists are located. Such places as Shiraz and Yazd can bake in the summer, although it is a dry heat. For those who either don’t mind the heat or actually like it, the summer is a great time to tour Iran because there are fewer tourists there.
In the winter season, the North of Iran and the nearby mountains can be quite cold. However, if you would like to go skiing in the outskirts of Tehran, winter can be a good time to go.
Another option for travel during the winter is to avoid the North of Iran and Tehran altogether and fly directly to Shiraz and concentrate on the great historical sites in the south. Again, the advantage is that there are few tourists and the Unesco World Heritage Sites are fairly empty.
All that being said, most people will want to tour Iran in the spring or the fall.