The best time to visit Isfahan is in spring, when the nights are warm but the daytime temperatures are still bearable. From mid-June,, the climate becomes really hot and visits to the city are less pleasant.
Spring is the most pleasant time to visit Tehran. The New Year period is very unique, as the city's inhabitants leave town for 2 weeks and several museums close. In winter, the weather is cold and grey, but the ski slopes are very close for those who love to ski!
It's best to visit this region in April/May, with temperatures and humidity rates still bearable. To discover the region's forests in their most beautiful finery, choose autumn.
It's most pleasant to visit Tabriz in May or June. You should avoid the winter months as temperatures can drop to -30°C.
The best time to visit Shiraz is in spring, when the nights are warm but the daytime temperatures are still bearable. The city's gardens are covered in flowers and buds. From mid-June, the climate becomes really hot and dry.
As the central part of the country and Yazd are desert regions, opt to go there right at the start of spring. The nights can still be cool, but temperatures will be bearable in the day.
As with the rest of the country, Mashhad should really be visited in spring. It's best to avoid the Shiite religious festivals (which follow the lunar calendar and change depending on the year), as the city's main attraction then becomes inaccessible.