Xichang's climate is genuinely unusual for Sichuan — warm and mostly sunny year-round, with annual sunshine hours comparable to Kunming. Winter temperatures rarely fall below 5°C in the valley and the dry season from October to April brings reliably clear skies. This makes Xichang an appealing destination precisely when the rest of Sichuan is wet, cold, or overcast.
Summer (June through August) brings the Yi Torch Festival and most of the year's rainfall, though Xichang's rainfall is modest compared to the province's northern mountains. The festival period in late July books out accommodation city-wide; plan accordingly if that is why you are coming. Spring and autumn are the most comfortable seasons for outdoor activity, with excellent visibility for the lake cycling route and mountain views.