Here’s the logo for Fête de la Musique. It’s in French. Here’s a Wikipedia bit in English. Note all the links at the bottom.
The French site reports it started in 1982; Wiki says the idea was “first broached in 1976.” Whichever is correct doesn’t matter. What does is that it happens every year, three decades worth, around the globe. Find out what’s happening in your country and city.
To participate officially, and be sanctioned by the French organization, there is a PDF download listing the requirements. It includes all performances be free to the public, all musicians donate their time, professional and non-professional musicians may be involved, and all noise regulations must be observed. The web has translators so all can get the gist of the French text.