The best time to visit Karakorum is June through August, when the steppe is green, the weather is warm, and ger camps are open and fully staffed. Summer days can be surprisingly hot — temperatures regularly exceed 30°C in July — but nights cool off sharply and the air is dry. This is also peak season, and the monastery and museum will be at their busiest, though they never feel truly crowded by international standards.
September is an excellent shoulder-season option. The grasslands turn gold, the light is lower and more interesting, and the tourist infrastructure is still in operation. Naadam — Mongolia's national festival of horse racing, archery, and wrestling — falls in mid-July and is celebrated in Kharkhorin as well as Ulaanbaatar; if your timing allows it, this is one of the better places to watch the festivities without the capital's crowds.
Spring (April–May) can be cold and unpredictable, with the possibility of snow and sudden storms. Winter is genuinely harsh — temperatures drop well below -20°C — and most ger camps close entirely. Travel to Karakorum in winter is possible but demands experience, proper equipment, and advance planning.