You've mis-parsed it. Anyhow, here it is in the active voice "Every year, they hold an exhibition of the photos that they took. " That is, they exhibit the photos.

at the end of each holiday /an exhibition is held (of the photographs taken) / so that ..... an exhibition is held it is about the photographs. and the photographs are taken in the holiday people can show their work(by photos ) to clients and receive feedback

"the exhibition is held" means that the exhibition took place. "The exhibition" is a collection of "photographs taken".