The festivals and concerts in Iran are a bit different from those in the west.

The concerts are mostly traditional or pop. Every month, there are a number of concerts mostly in the capital Tehran or in major cities like Isfahan and Shiraz.
The concerts are not announced that much in advance so you have to wait a few more month and then choose one of the concerts in September. Most concerts in Tehran are held in Vahdat or Roodaki hall so you may the info accordingly.

The festival are more limited. The most important one is Charshanbeh Souri in March. The second important one is Mehregan, an ancient Zoroastrian custom similar to Thanksgiving.
It is much older than Thanksgiving. It is at least 25 centuries old but it is observed in the country and in some villages. The date must October 8