Hello! I'm trying to find information on train and metro availability on May 1st. Ideally i'd like to take the train to Giverny for Monet's gardens and then return. Thanks for any advice or recommended sites to research.
Trains run on holiday schedules as usual. Use the SNCF site or Trainline.eu to identify train schedules for Giverny and make sure Giverny is open on their own fondation monet website. Many, perhaps most sites, are closed May 1. Perhaps you have already found that Monet's gardens are open, but if not, be sure to check.
The Paris metro also runs normally on that day.
Note that there are several French cities with zero public transport on May 1 (even as large as Marseille and Lille), but Paris is not one of them.
Nice has no public transportation on May 1. I was very grateful to the taxi driver who showed up to work that day.