Iran has a wide range of climates — from the subtropical Caspian coast to the parched central plateau to the cold northwest — and the best time to visit depends on where you are going.
Spring (March–May) is the most popular time and the best for most of the country. Temperatures are mild across the plateau, wildflowers appear in the mountains, and the light is good. The major caveat is Nowruz, the Persian New Year, which falls around March 20–21. For the two weeks around Nowruz, Iranians travel in huge numbers, accommodation is scarce and expensive, and some historical sites become very crowded. Book well in advance if traveling over Nowruz, or avoid it if you can.
Autumn (September–November) is the other excellent season. Temperatures drop to comfortable levels, the summer heat has passed, and crowds thin out. This is a good time for Isfahan, Shiraz, and Yazd. The Caspian coast stays green and pleasant into October.
Summer (June–August) is brutal in most of the country. Temperatures on the central plateau and in the south regularly exceed 40°C (104°F). The desert cities — Yazd, Kerman — are at their harshest. The exceptions are the mountains around Tehran and the ski resorts of the Alborz, which are pleasant in summer and used for hiking. The Caspian coast is warm and humid but not extreme.
Winter (December–February) is cold in the mountains and northwest — Tabriz and Ardabil can be genuinely frigid with heavy snow. The south (Shiraz, Bandar-e Abbas, the Persian Gulf islands) is mild and often the best time to visit these areas. Tehran is cold but usually manageable. The Dizin and Shemshak ski resorts near Tehran operate December through March and attract domestic visitors in large numbers.
Ramadan falls in a different part of the solar calendar each year (shifting about eleven days earlier annually). During Ramadan, restaurants may be closed during daylight hours, and some attractions have adjusted hours. Travel is possible during Ramadan — the evenings come alive — but it requires more planning around food and timing.