It's a national holiday. Whit Monday. Banks are closed. What about major museums, Ste. Chapelle, and the like?
Major museums will be open if Monday is a regular opening day. As for other "sites", Google the ones that interest you for info.
There are three days most (but not quite all) museums are closed. Jan 1, May 1 and December 25. Other holidays are rarely observed with museum or even shop closures. You can always look on the websites to see days they are closed besides the one day a week most museums close.
Open: Louvre, Ste Chapelle (closure between 1:00 - 2:15), Centre Pompidou, l’Orangerie
Closed: d'Orsay, Rodin, Carnavalet, Versailles (normal Monday closure for all)
Thanks, folks. I get the picture now.