Taya pointed me to Software Freedom Day on September 10th:

Software Freedom Day is a global, grassroots effort to educate the public about the virtues and availability of Free and Open Source Software. Over 200 teams are registered so far, and they have plans to celebrate Free Software at schools, universities, parks, and many other public places.

As it happens, it is also my birthday :-)

There is also an effort to organize the event in Finland.