This morning the number 2 union representing air traffic controllers filed a strike motion for May 1; therefore, later in the day, the French civil aviation authority (DGAC) ordered airports across France to reduce the number of flights that will arrive and depart from their sites.
Press is reporting that the DGAC asked the following airports to cut 25% of their flights on May 1: Paris-CDG, Nice, and Beauvais.
Most other airports have been asked to cancel 33% of their flights that day.
Please note that usually long-haul flights (such as to/from the US or Canada) are not the ones cancelled; it's usually flights within France or to/from other European airports.
The airlines will now spend the next couple of days figuring out how to redo their schedules to comply with this order; so if you have plans to fly on May 1, be looking for communication from your airline maybe Friday
Here is a press article from Le Figaro that you can run through Google translate if you want: