In fact, the Tibetan climate is not as harsh as many people imagine it to be. It is suitable for travel to Tibet from April to the beginning of November, and the best time is August and September. But if you only stay in Lhasa, you can go there anytime of the year.
Sun radiation is extremely strong in Tibet. The sunlight in Lhasa is so intense that the city is called Sunlight City. The thin air can neither block off nor retain heat so that the temperature extremes can be met in daytime and the same night respectively in Tibet. However it is not impossible to visit the holy snow land. May, June and September are the tourism season in east Tibet.
Most annual rainfall comes in the rainy season that starts from June to September. Usually it rains at night in Lhasa, Shigatse and Chamdo area. The rainfall may block roads and make travel difficult but the scenery at the time will be the best.
Here is some more specific information in different areas:
Lhasa: Lhasa is suitable for visiting all year around due to the comparatively lower altitude and warm weather. If you are adapted to the chilliness, try to come here in winter for the cheap cost of transportation.
The border of Nepal/China: The Friendship highway is basically in good conditions year around. But from December to February, the thawed road could make some trouble Besides, try to avoid August - landslide could happen in the rainy season.
Mt. Everest Region: Early May and early October are the best time to visit Mt. Everest. Due to the clear weather, you have great chance to see Mt. Everest's true face. From December to February, it's too cold to go to this region. But the magnetism of Mt. Everest always attracts people anytime of the year.
Ali (Mt.Kailash): Even without climate restrictions, this area is already inhospitable. Big rain and snow could make the journey worse. However, for those determined tourists, the appropriate time is May, June, July, September and October.
Eastern Tibet: Don't go to this area in July or August, the rain could ruin the road, and make terrible landslides. While in winter, the road could be frozen.
Northern Tibet: With the average altitude of 4,500m, this area offers very limited time for tourists. Summer is the prime time to enjoy the great plain in northern Tibet.