Central Tibet tour can be done round the year however the best time to visit Tibet is April - October.
March: Every year during the month of March Tibet will be closed to foreign tourist. This is an annual closing. The anniversary of the Dalai Lama as well as many uprisings and protests happens during March in the past so Tibet permit normally stopped being issued for the month of March.
April - May: April - May is a good time to visit Tibet. The weather is generally clear and the temperatures are crisp, but not frigid and there are not too many tourist.
June - August: Summer is a warmest time in the Tibet, but still snow fall is common in norther region above 4000m. This is the most popular time of the year to Visit Tibet. Most of the Tourist travel to Tibet during this time period. It is a peak season in Tibet and every thing such as hotels, transportation, train , flight every thing gets expensive.
September - October: The fall is also equally good time to travel to Tibet. The skyes are clear and mountains are snow capped. Temperate are mild as the summer but are still comfortable. In the high mountain area temperate will be freezing so appropriate clothing is recommended.
November - Feb: It is a winter time. It will be quite freezing during this period of time. Central tibet can be explored during this time of year but norther part of the tibet is quite tuff.