the website does mention something about "weekends only":
http://en.parismuseumpass.com/rub-faq-16.htm
"The PARIS MUSEUM PASS includes access to all tours at the Palace of Versailles, open according to the season* and the audio tours. Please note, however, that the following attractions are not included: performances, the "Grandes Eaux Musicales" fountains and musical gardens, conferences, bar/restaurant services and transport.
To visit the gardens and the "Grandes Eaux Musicales", an additional entry charge must be paid.
- i.e.: the "Grands Appartements", the Hall of Mirrors, the King's Bedchamber, the Queen's Bedchamber, the Dauphin and Dauphine apartments as well as those of Louis XV's daughters - weekends only - exhibitions, the Trianon Castles and the Estate of Marie-Antoinette."
This seems to apply to rooms which are only available to tour seasonally; specifically the apartments of the daughters. I've seen mentions that those rooms are only open during weekends in summer but I'm not finding that info on the site. I do see that because the Queen's Apartments are currently closed, that visitors are being routed through the Mesdames’ apartments .
http://en.chateauversailles.fr/news-/events/autres-evenements/travaux-du-grand-appartement-de-la-reine-en