Not long ago, I went to the Yves Saint Laurent exhibition at the Petit Palais. Seeing more than 300 of YSL’s stunning creations would have been enough, but there was an added bonus that made the evening even more special: the doors stayed open until midnight….
from Paris Museums at Night | Girls Guide to Paris.
Note: you can find a very complete list of all the Paris museums opening till late, arranged according to the day of the week on this site: http://quefaire.paris.fr/articles/162
This is in french but quite easy to understand + there are links to the Museums’ websites (with english versions).