Mountaineering season in sikkim starts from September to November and April to June. when daytime temperatures at most altitudes are comfortable. However, the temperatures of the southern part of the Himalaya are somewhat hot. Above 4,000m in the mornings and evenings the temperature will be considerably lower and can drop to -20 Degrees Celsius. Usually daytime temperatures are pleasant but nights are very cold above 5,000m. Temperatures are milder in spring but there can be more snow around. Late April , June through last week of sep. to first week of Dec. is the safest and best time for climbing and mountaineering as well as trekking too.
ALTITUDE SICKNESS: -
Many people who trek in the Himalayas experience the effects of altitude especially if climbing to 3000m and above. The problems arise from the inability of the body to deal with a rapid gain in altitude. Therefore, it is important to follow the recommended guidelines in terms of meters of ascent per day and to allow adequate time for acclimatization.
CLOTHING AND EQUIPMENT: -

With a big range of temperatures and conditions you must be prepared for anything.