The official Fête de la Musique was created in 1982 by Jack Lang, a French politician and Secretary of Culture at this time.
That day, everybody is allowed to go on the street and play an instrument: professional musicians and amateurs play music, sing, scream, shout etc., in a happy cacophony. Concerts are organized in the whole country, and are free of course. Every park, every street corner, every public building hosts an exposition, a concert, a demo, where neophytes can listen, practice, participate.
Since 1985, la Fête de la Musique is also celebrated in Europe, and has been exported to different continents after that. Here are some of the posters made worldwide to announce the festivities. You can see more at la Fête de la Musique webpage.
So... let's rock!