Free from March to May

I'm thinking of going to Sikkim between March and May, but I'm not sure what the best time would be. I've never been to the north-east, so I'm thinking of a 10-12 day trip. I'd like to visit Darjeeling as well. Could you give me some advice on the best time to go? It'll be helpful if the roads are open, as Google says it's the beginning of the monsoon in the north-east. Any suggestions would be great.