The French are the butt of many jokes, but given credit where credit is due, and it was indeed two Frenchmen, Jack Lang and Maurice Fleuret, who came up with the idea for a world Music Day, or Fête de la Musique, way back in 1981. In Slovenia it's known as Praznik Glasbe (Music Holiday or Music Festival), and in Ljubljana especially you'll find live concerts taking place in outdoor spaces all over the city. More info from the official site below.