Hi,
Sikkim is one of those places that can be visited any time of the year. The weather then will be pleasant during the day, yet chilly at night so do carry some warm clothes as well! Summer is considered to be the best time to visit Gurudongmar Lake, Yumthang Valley, Rumtek Monastery and other main attractions in Sikkim. Go through more articles on Tripoto to get an idea of the best places to visit in Sikkim. Attaching a few for you to check out! Hope this helps.