October to March is the best time, with cool clear days, cold but tolerable nights, and the post-monsoon mountains at their greenest. December and January can drop to around 5°C at night; bring a jacket.
April to June is hot — 35–40°C, though still cooler than the plains. The pre-monsoon dust storms are unpleasant. The monsoon (July–September) brings heavy rain that occasionally closes the mountain roads to Mussoorie and the higher hill stations, but it also turns the Doon Valley impossibly lush. If you like rain on the windowpanes, this is the time.