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Sziget Festival
This is the biggest festival of the entire Central and Eastern Europe.
Date for 2009: August 12-17
Website: http://www.sziget.hu
Check out the Sziget Fesztivál website for a bunch of other festivals included spread through the Summer. They include:
Hegyalja Fesztivál
Classical rock festival with Hungarian and international bands and artists.
Date for 2009: July 15-19
Website: http://www.hegyaljafestival.hu/