Tehran is definitely not a destination that you may want to visit at any old time of the year. Tehran's climate can be very harsh to tolerate in the summer with degrees averaging around 84 F (29 C). Winters are very cold and can snow a lot especially in Northern Tehran. It can go as low as 10 (-13 C) .Extreme weather, the heat in the summer and the cold in the winter can make for an unbearable trip and those affected by it, should definitely only travel during fall or spring. 

Safety is the other major issue regarding when to travel to Iran. Unfortunately, Iran is on the danger list by the U.S. Department of State, as an unstable destination. Large demonstrations regularly take place here and there is an anti-Western and anti-American sentiment. Depending on the global climate, you will want to consider these factors when planning your trip, always checking with the American or your government along with tour agents.  At times, American, Canadian, and British governments have gone as far as to say that traveling to the Tehran International Airport is not even recommended at all; therefore you must be current with the global climate and Iranian relations with your home country.

A noteworthy event to see while in Tehran, is the Tehran Short Film Festival held in November. Although you may not understand most of the films, the experience will be culturally enriching.