The best time to visit Lhasa for sightseeing tours is during the spring (April to June) and autumn (September to October) months. During these periods, the weather is generally mild and clear, providing great conditions for outdoor activities and viewing the city's landmarks. The summer months (July to August) can be pleasant too, but they are also the monsoon season, so you should be prepared for occasional rain. The winter months (November to March) are cold, but Lhasa often enjoys sunny days, and the tourist crowds are smaller, so it can be a good time for those looking to avoid peak season.