Tuesday is national museum closing day in France. Many state-owned museums are closed on Tuesdays, as are many local museums outside Paris. In Paris, most city-owned museums are closed on Mondays. Some museums are open later on one night a week, usually Thursdays. Most museums close on some public holidays but not all. There are many exceptions, so always check before you go.
I think the reason why Tuesday was picked as a closing day is that normal people should be able to visit museums, so they are open on Saturdays and Sundays. Many shops are closed on Mondays, so museums are open that day for the sake of people who work in shops. In Paris, most shops are open on Mondays, so there is less point in opening the museums for their sake.
Here are a few popular attractions¹ that close on Tuesdays:
Here are a few popular attractions that close on Mondays:
Here are a few popular attractions that open 7 days a week (except holidays):
This site has a list of museums in Paris with addresses, opening dates and times, and prices. At a glance, it looks reasonably accurate.
¹
Roughly, the top visited museums according to Muséostat.
Credit:stackoverflow.com‘