Music Day is a connected set of free public events which takes place each year on the 21st June, our midsummer. Each year there are events in over 120 countries and 700 cities around the world, not including France where there were too many events to count!
The event is also known as Fête de la Musique(www.fetedelamusique.culture.fr) (or World Music Day) and originated in France in 1982. Since then the idea has spread across the globe with each event holding to the same simple set of principles: that the day is held on the mid-summer solstice and that all concerts and events are free and accessible to the public.
Typically this means that the music is brought out onto the streets, though venues are also encouraged to get in on the act!