The two main seasons for visiting Khatgal are summer (June to September) and winter (January to March). Summer is the classic horse-trekking season: the trails are open, the weather is warm enough for camping, and the lake is ice-free for boat trips. July and August bring the most visitors and the best weather, though nights are cool even then.
Winter has its own appeal — the frozen lake, ice fishing, and the dramatic cold-weather landscape — but temperatures can drop to -35°C or lower, and you need proper gear and experience. Spring and autumn are transitional seasons when the trails can be muddy and transport uncertain.
The Naadam festival in July is celebrated in Mörön but not in Khatgal itself; if you want to combine the lake with traditional Mongolian sports, plan around mid-July.