Tibet is not close in September, so far there is no official notice. If you are booking with the real local Tibet travel agency you can get the permit.
Tibet is closed from September 1st 2015: Tibet tourist bureau is not accepting any application for Tibet travel for the month of august and there are asking us to change the itinerary of any tour in the month of august where we have departure in the month of September. beside, this no any travel agency in Tibet has any update about it. That means Tibet is closed from September 1st 2015.
All the international travelers visiting Tibet in the month of august has to leave Tibet before august 31st. rumor has it that the closure is for the celebration of 50th year anniversary of the founding of Tibet Autonomous region of China.
We are not sure when, Tibet will be reopen back for travelers and if we do have information we will update.
There is another date travelers need to keep in mind, This year annual yogurt festival starts from august 14th and Tibet tourism bureau is likely to be close for about four days for the festive holiday.