October through June is the opportune time to journey up to this part of the Himalayas. In December and January, Darjeeling, particularly, gets quite cold, and snow will hit the higher reaches of Sikkim. You will need to pack lots of warm wear. The monsoon season, when landslides occur, is not a great time to visit Sikkim. Navigating mountain roads are all the more difficult when they are wet and slippery.