May 1 along with Dec 25 and Jan 1 is the most reliably 'closed' day in France and most of Europe. If you look at websites those three days will be listed as closed on most museum and other attraction websites. There is still plenty to do and many restaurants and cafes will be open. But shops, most museums etc will be closed. The Eiffel Tower, Seine cruises, Jacquemart Andre and the parks are open, so there is plenty to do.
On my first trip to Europe with my husband over 30 years ago we landed in Venice on May 1 being totally ignorant of its holiday importance. I still remember us lugging our bags (no wheels then and first trip so we overpacked) over a mile over bridges and canals to reach our hotel since the boats were not running. Paris will be easier -- the metro and buses will be running and there will be plenty to do although much will be closed. Look also for demonstrations which are fun to watch.
janettravels44
www.janettravels.wordpress.com