Fête de la Musique is a free music festival taking place on the longest day of the year, June 21st. Its purpose is to encourage both amateur and professional musicians to perform on the streets together, and basically everyone have a groovy time. These all night extended music block parties have been going on in France since 1982 and now can be found all over the world, even in Pasadena!
It's a wonderful thing really. Sometimes known to get out of hand, but for the most part, people are there to have a good time.