When is the officially celebration in Tibet Autonumous Region?

If you refer to the establishment of the autonomous region celebration, it’s Sep. 1, so some activities will be held around that period. If for the Chinese National Day holiday (from your travel date), it’s Oct. 1-7. Neither of them would influence your trip there. Just apply for Chinese visa and get a Tibet Travel Permit through a travel agency. It is fine to travel then with a guided tour.