Traveling year round: moderate summers & cool winters

Yunnan's mild climate is both pleasant and diverse. The province enjoys a tropical highland humid monsoon climate, characterized by cool summers and warm winters. The annual average temperature lies between 13-20°C; January average temperatures range from 8°C to 17°C, and July averages vary from 21°C to 27°C. The annual rainfall is 1100 mm, with over half the rain occurring between June and August.
The fair weather is the result of the province's location on south-facing mountain slopes, receiving the influence of both the Pacific and Indian oceans. The plateau region has moderate temperatures. The western canyon region is hot and humid at the valley bottoms, but there are freezing winds at the mountaintops.

Always just like spring

In Kunming it always feels like Spring. In Dali it is, despite it's altitude of 2000 metres, most often warmer. Sometimes a cool fresh wind blows through the area. This is the result of Dali's location between the beautiful Lake Erhai and the Cangshan mountains, rising up to a height of 4200 metres (1300ft).
Lijiang is located at an altitude of 2200 metres. It can get quite warm during the day, but in the evening it soon turns chilly. In Zhongdian - also called Shangri-La - the sun may be even stronger than in Lijiang. But, because of the town's altitude at a height of 3200 metres, and its situation close to Tibet and the Himalayas, warm clothing is advised for evenings and nights.
The southern part of Yunnan (Xishuangbanna) enjoys sub-tropical to tropical temperatures.

Spending winters in Yunnan

Yunnan also is very beautiful and fairly warm during winter. In January the trees are full of flowers, with sometime some snow left on the tops of the mountains.
In fact, there is not such a thing as "Best time to travel" applicable in Yunnan. It is actually very pleasant all year round.