First saturday happens every month, on first saturday at La cité des Sciences de la Villette, in Paris.
See website (in French) at http://premier-samedi.org
It's not an install party, the main objective being to provide users with advices on using their distributions (be it Ubuntu, Debian, Fedora, Mageia, you named it...) and using software, solving their day-to-day difficulties (mail, web, multimedia, advanced uses, sometimes bug) and getting them to master the distribution of their choice and its intricacies.
You may see it as the "service après-vente" (corresponding, in english, to user support in the long run: suffice it not to install GNU/Linux once, but being efficiently used afterwards, whatever the distribution, it's Libre Software). As advanced users, we choose to provide support, distribution-agnostic mostly, we may be recognized as most competent, as we can even support distributions we don't directly contribute to :-) (that's libre software, let's make people happy about it, using it and deploying it, either through Mageia or through their own ways).
Regularly, around 30 advanced users (not all using Mageia yet!) and 30 requesters meet, to get answers and spend a good time together, around libre-software and solutions for day-to-day work with computers, people are from 7 to 91 years old, with 20 years or 1 month experience with GNU/Linux. Conferences and formation happen at the same time, Blender User Group for example, or sometimes RMS.