The Musee d'Orsay will be free on the weekend of Friday, 2 December 2017 to Sunday, 4 December 2017. Entrance is by Porte C, around the corner from the admission line.
Attention: Friday, the museum will close at 19h00 - allow 15 minutes to clear out.
Sunday, the museuem will close at 18h00 - allow 15 minutes to clear out.
There will probably be a lot of people going to the Museum, so I'd go as early as possible. No tickets necessary.