The best time to visit Mussoorie is late February to June, and again September through November. March and April are pleasant and the hills are in bloom. May and early June, when the plains bake, are peak season — Mussoorie fills with Indian families escaping the heat, hotels charge premium rates, and the Mall is uncomfortably crowded. Book accommodation well in advance if you plan to visit in May.
The monsoon arrives in July and stays through August. The views disappear entirely into cloud, and occasional landslides can disrupt road access. It is quieter, dramatically green, and romantically atmospheric if you don't mind the rain.
October and November are arguably the best months: the monsoon has cleared, the air is sharp, and the snow peaks to the north are frequently visible in full. Temperatures are cool to cold. December through February brings frost and occasional snowfall; the town slows down but a dusting of snow is appealing if you're prepared for it.