The best time to visit Kalimpong is October to November, when the skies are clear after the monsoon and Kanchenjunga is visible from Durpin Hill on most days. March to May is the second-best window: rhododendrons are in bloom in the surrounding hills, and the weather is mild without the heat and haze of early summer.
Avoid June to September if you can. The monsoon brings heavy rainfall, frequent landslides, and persistent mist. Roads can be cut off for days, and the Kanchenjunga views disappear entirely. The flower festival is held in October, timed to coincide with the most pleasant weather and the flowering of gladioli and dahlias.
December and January can be cold, with occasional frost at night, but are generally dry. The views are often excellent in winter, and the town is quiet. Spring, when orchids bloom in the nurseries around town, is also a good time to visit if you're interested in horticulture.