The best time to visit is spring (April–May) or autumn (September–October). In both seasons the temperatures are mild — roughly 15–25°C — and the landscape is at its most pleasant. Spring brings greenery to the desert margins and wildflowers to the steppe. Autumn is golden and dry.
Summer (June–August) is brutal. Ashgabat regularly hits 40°C and above; the Karakum Desert is hotter still. The Darvaza crater, glowing against the night sky, is one reason people still come in summer — but do all outdoor activity early in the morning.
Winter (December–February) is cold in the north, milder in the south. Ashgabat can freeze at night but is usually tolerable during the day. The ancient ruins of Merv and Kunya-Urgench are perfectly pleasant to visit in winter and are even more deserted than usual.
Nowruz (the Persian New Year, around 21 March) is the biggest celebration of the year. If you can time a visit for late March, you will see the country in a festive mood — a rare glimpse of public life that is normally quite muted.