Best Time to visit Sikkim is October to December and March to May.
Below is the itinerary we followed when we visited Sikkim:
Day-1:Flight to Bagdogra and reached Siliguri
Day-2:Siliguri to Gangtok and applied for necessary permits.Sight-seeing in Gangtok.
Day-3:Gangtok to Lachen.
Day-4:Gurudongmar Lake and Kaala Patthar. Reached Lachung by evening.
Day-5:Yumthang Valley and Zero Point.Back to Gangtok.
Day-6:Nathula Pass and Tsomgo Lake.
Day-7:Char Dham in Namchi. Reached Ravangla and visited Buddha Park.
Day-8:Rabdenste Ruins and back to Siliguri.