Jan end to May end is the best time to visit. In winters there are few places for which the roads are closed due to heavy snowfall. Even till early March you will be able to see snow around Lachung and Nathula pass. After May it is not advisable to visit as there is chances of landslide and road closures during monsoon
Sikkim is best visited between March and May, or between October and mid-December. If you want to see Gangtok's flowering natural beauty, the ideal time to come is in the spring, from March to May.
As suggested by Sikkim Tourism, the best time to visit Sikkim is either between March and May or October and mid-December. If you want to witness the blooming natural beauty in Gangtok best season to visit would be in spring, from March to May.