Most guidebooks tell you to avoid the Northeast Himalayas during monsoon. That advice is not wrong, but it is incomplete.
The trade-offs
Roads to North Sikkim can close for a day or two after heavy rain, so build slack into your itinerary rather than a tight day-by-day plan.
The upside
Waterfalls are at their fullest, the terraced hillsides turn a deep green, and popular viewpoints that are elbow-to-elbow in October are nearly empty.
Practical tips
Book a cab package with a driver who knows the current road status, keep 1–2 buffer days, and carry a rain shell rather than an umbrella.