Not only is it free Museum day on Sunday, but it is also Nuit Blanche on Saturday.
If you haven't come across Nuit Blanche before, it is (for me) one of the highlights of the year in Paris.
Nuit Blanche (which literally means White Night) is an all night art and performance event. It starts at 7pm on the Saturday and runs right through the night to 7am on the Sunday. All over the city (normally clustered around specific areas) there are art installations. And it is art in its widest context - from large orange yetti's in a luminous blue room, to all night classical piano concerts, through sculpture, audio-video, and somethings that are just weird (like last year where you could stand on a strip in front of a wall, you would be projected up on a screen in a farmyard setting - although it wasn't you exactly, you got transformed into a pig, or a horse, or a refrigerator, or a yellow VW Beetle, and these things move as you do around the strip).
The official site for the Paris Nuit Blanche 2012 is http://nuitblanche.paris.fr/ and there is also a free smartphone app for Andriod and Iphone.
Some of my photos from the 2011 Nuit Blanche are below.