From April to November is the most perfect time to visit Tibet. Yet, the place you visit affects your decision as well. Lhasa, Tsedang, Gyantse, Shigatse and Nyingchi, lower altitude areas, are suitable for visiting all year round, as the weather here is mild. From December to March, the season of winter, there are less tourists, therefore the costs will not be as high as the rest time. Also, if you come to Tibet in winter, you can have better opportunity to see Mt. Everest clearly.
July and August are the rainy seasons in Tibet, mainly night rain only which usually won't affect your sightseeing in the day time but will bring some difficulty to trekking tours. This two months could see about half of the annual rain in Tibet.
April, October and November often bring some dazzling clear weather and you could feel very comfortable in the lower altitude places.
For some popular tours, you may need to note the following weather restrictions:
Besides the above climate issues you shall consider when planning a tour to Tibet, you need to get prepared for the strong sun radiation and big temperature difference between day and night for traveling there as well.
Which Season Should You Travel To Tibet