Mongolia has a harsh continental climate — one of the most extreme on earth. The difference between summer and winter temperatures in Ulaanbaatar (the world's coldest capital) can exceed 70°C. This makes the timing of your visit important.
June to August is when most travelers visit. Days are long and warm, the steppe is green, and ger camps are open. July is the peak month, driven largely by the Naadam Festival (July 11–13). Nights are cold even in summer — bring layers. July also brings occasional heavy rain.
September is excellent and often overlooked. The weather cools, crowds thin out, and the landscape turns gold. The Golden Eagle Festival in Bayan-Olgii takes place the first weekend of October — early September is a good time to head west in preparation.
May can work if you come prepared for cold nights and the possibility of snow. The steppe starts greening in late May, ger camps begin opening, and there are no crowds. Cheaper and quieter than peak summer.
October to April is for the adventurous. Winter in Mongolia is genuinely brutal — January temperatures in Ulaanbaatar regularly hit -30°C or below, and the countryside can be impassable. That said, the winter landscape is extraordinary, naadam-style ice festivals happen on frozen Lake Khovsgol in February or March, and you'll have the country largely to yourself. Come only with a specialist operator who knows what they're doing.
The Gobi Desert has its own logic: summer temperatures there reach 45°C, making it more comfortable in late April–May or September–October when it's warm but not punishing.