Prague has some great stuff happening in September!
To start off with there's the celebration of St Wenceslas (patron saint of the Czech Republic) from the 14/09 - 29/09. You can expect some great local market stalls as well as live music and other entertainment held at the aptly named Wenceslas Square.
Moving right along to the major concert of the month. Coldplay grace their presence at the Sinot Tip Arena on the 16/09.
A different type of music, but just as popular is "Dvorak's Prague Festival" from the 8/09-22/09 celebrating the great Czech composer, Antonin Dvorak. Both Czech and international orchestras will feature at the Rudolfinum Concert Hall.