Keylong's season is dictated by the mountain passes. The Rohtang Pass connecting Keylong to Manali typically opens in late May or June and closes by late October or November. The Baralacha La pass on the road north to Leh is passable only from late June to September — if you are continuing to Ladakh, check road conditions carefully. The Atal Tunnel, opened in 2020, now provides year-round road access from Manali, which has changed the town's character somewhat.
The best months to visit are July, August, and September, when the weather is clear and warm by highland standards, the passes are open, and the gompas are active. Monastery festivals (Chham dances) are usually held in June or July — ask locally for exact dates as they follow the Tibetan lunar calendar.
Winter (November to May without the Atal Tunnel) cuts Keylong off from the south, though the town continues its quiet life. The cold is severe — temperatures drop well below freezing at night — and most traveller facilities close.