North east side summer has started ending, while monsoons are slowly setting in. Rains have started in Sikkim that 20 to 25 days of rain is predicted. It’s an ideal weather to travel for monsoon travellers. You will see more of lush part of Sikkim, but your that days travel will be based on that days weather.
My relative travelled recently to Sikkim and stayed for 10 days out of which 6 days rained, though not much heavy.