The central plateau of Iran is marked by hot and dry summers and sporadic rainy winters. The Desht-e Kavir, southeast of the capital, is harsh, inhospitable , and very, very hot in summer. Winters are not much better, and at night the temperature can fall well below zero. If any time of year can be called pleasant in this salty wasteland, it would have to be between October and December. In the far southeast of Iran, away from the Persian Gulf proper, temperatures are a little lower.
Summers are hot and dry, winters mild and dry.
Down in the south of Baluchestan, along the coast of the sea of Oman, the climate is similar to that of the Persian Gulf region, or even hotter, with strong winds in summer. There is very little rain throughout southeast Iran and frost would be a great novelty. Spring and autumn are the ideal times to tour Iran, but summer or winter can be OK, so long as you do a little planning and take a few precautions. The northwest of the country is generally the coldest and among the rainiest parts of the country. Snow frequently remains until early spring, or even later in the mountains.