Nah it's not only trumpets, if I remember well... It's like a huge concert/competition with trumpet bands from all around the country, plus a huge drinking party (loads of rakija, mostly plum) and a lot of foreign people from friendly countries come aswell.
Well i'm a little too young to drink but technically I could since we don't have a legal drinking age I spent the time enjoying the sound of trumpets and trying to communicate with Greeks xD