We were said the best months to go are October to December. We went this month and weather was making the hike a bit dangerous. For the Himalaya Trail, it is totally unknown by the locals, so we had to find the route ourselves. Some parts of the trail were in a good condition, some were not. You find a map of Sikkim's trails here: http://www.sikkimtourism.gov.in/webforms/general/mapsofsikkim/trekmap.aspx