Mussoorie is a hill station which has pleasant climate around the year. It is a very cool place with greenery at its full bloom during September to November. The best season to visit Mussoorie is between April to June and again during September to November. When the plains of North India experience heat waves, Mussoorie provides a welcome break. September - November is spring time here. During this season Himalayas are clearly visible.
It is better to avoid monsoons as the roads to Mussoorie are in bad shape. Same thing applies for the winter when roads are blocked due to snowfall.