The Salzburg Festival offers a big variety of premium concerts, theatres and operas. You can find the detailed programme here: http://www.salzburgerfestspiele.at.
But despite the changing programme, there are some highlights that say the same. One of them is the beloved play ‘Jedermann’ (Everyman) of Hugo von Hofmannsthal, that premiered at the first Salzburg Festival in 1920 and was played every year since then. Also the festival ball in the last week on August 31st is a popular festivity.
For those who do not have tickets, there are the Siemens Festival nights. In the city center on the Kapitelplatz a lot of the plays and concerts are shown on an open-air cinema screen. This enables you to enjoy current Festival productions for free and in front of the special scenery of the city center and the fortress.