Chengdu is overcast for much of the year — the Sichuan basin is notorious for its "four seasons in one day" weather and limited sunshine. Spring (March–May) and autumn (September–November) are the most pleasant times, with moderate temperatures and occasional clear days when the snow-capped mountains to the west become visible.
Summer (June–August) is hot and humid; winter is mild but damp and grey. Visiting Qingcheng Shan or the panda base in winter has the advantage of smaller crowds, and the pandas are often more active in cooler temperatures. Avoid the Golden Week holidays (early October and late January/February) unless you enjoy enormous crowds.