Actually if you like to see more greenery than it is good to visit to Tibet on August/ Sepetember. It is considered to be best season to visit Tibet. Espically holy mount. Kailash is good to visit at this time. Best time to visit Tibet is throughout the year. It is fun to visit Tibet in winter (December and January)as it is very cold. In this time of year we recommend you to carry sleeping bag and down Jacket with layers. Like wise when you travel in Tibet in June and July you will have rain which is again good. But it is necessary to carry raincoat and umbrella.