We were disappointed to find that Versailles was closed on May 1, a holiday in France. This is not mentioned in the Paris 2018 book concerning Versailles hours. However, May 1 is listed under the Holidays section in the appendix. Just be careful with planning!
Always check the official website of any place you want to visit. The guidebook has info, but not up to the minute.
May 1st is a very traditional holiday in Europe -- similar to Labor Day in the US.
In Europe, May 1 isn't just a holiday, it's one of the very biggest. For instance, the Prado in Madrid is only closed three days a year - January 1, December 25, and May 1.
For my recent trip, I deliberately planned to take the Eurostar from London to Paris that day, so I'd spend a lot of it in transit, rather than obsessing over all the places I wanted to see that were closed. For future reference, the Jacquemart-Andre Museum is just about the only thing in or near Paris that's open on May 1.