The best time to visit Tibet

The best time to travel Tibet is between April and October, and peak season is from May to September. The best months are May, June, September or October as July and August are rainy months. Those who travel in their own car or on foot should avoid the rainy season especially when entering Tibet along the Sichuan-Tibet Highway, and the sections between Lhasa and Nyingchi and between Lhasa and Ngari. There will be mudslides, cave-ins and mire on certain sections of the road, blocking the passage of vehicles. However, if you are only planning several days in and around Lhasa and getting there by air, there will be no problem for you to travel at any time from April to October.

Here is some more specific information in different areas:
Lhasa /Shigatse /Lhatse /Tingri /Nyalan: Along the Friendship highway is basically in good conditions year around. But from December to February, the thawed road could make some trouble. Try to avoid August - landslide could happen in the rainy season.
Mt. Everest Area: 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 (if you are lucky). From December to February, you'd better not to go to this area because it is too cold - except you are real Great Adventure People.
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 rainy season) because the rain could ruin the road, and make terrible landslides. 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 (July to August) is the prime time to enjoy the great plain in northern Tibet.