The best time to visit Mussoorie is from October to June.
From March-June the weather is pleasant to enjoy all sorts of activities in Mussorie.
From July-September it is usually not advisable to visit Mussories as these are the monsoon months and there are chances of landslides.
From October-February the weather is chilled as it snows in Mussorie. If you want to enjoy snowfall then December to mid-February is the best time to visit Mussoorie.