"When's the best time to trek in the Himalayas?" is a question we can't answer without knowing the region, because Himachal, Uttarakhand and Ladakh run on three different calendars entirely.
Himachal Pradesh treks like Hampta Pass open once the winter snow clears the pass, typically June, and run through October before the first serious snowfall closes it again. Peak season is July–September, which unfortunately overlaps with monsoon lower down — the trail itself sits high enough to mostly avoid it, but road access can get disrupted.
Uttarakhand splits into two very different seasons depending on the trek. Kedarkantha and Brahmatal are winter treks, December through April, built entirely around snow — the frozen lakes and summit snow are the draw, not an obstacle. Valley of Flowers runs the opposite calendar: July to September only, timed to the monsoon bloom, with the trek genuinely not worth doing outside that window.
Ladakh runs on the most extreme split of any region we operate in. Chadar happens only in January–February when the Zanskar freezes solid enough to walk on. Every other Ladakh trek and the region's road-accessible circuits run June through September, once winter snow clears the high passes into Leh.
The practical takeaway: pick the trek first, then let its region dictate your dates — not the other way around. Trying to fit Chadar into a June holiday, or Valley of Flowers into October, isn't a scheduling inconvenience, it's a different trek that doesn't exist yet.