Solidarity Day?

I don't work 'in" France so I don't follow their holidays. Yeah it's a bit weird when I'm like "I have Monday off for Memorial Day" and no one else is buying white sheets and going to the beach. Most things are open on Holidays, but I could have sworn they didn't used to be. Amazon, Uber and every other American company probably just doesn't observe those days either. I think one of the worst things about living here is trying to figure out which holiday is an 'open boulangerie day' and which one isn't. Or 'which one will make it hard for everyone to meet because the metro will close early' holidays. 

I usually have to use my trusty French holiday site when planning anything. Most expats are the same. But the Whit holiday doesn't make a lot of sense to me; some people work and some people don't? And the metro runs on shorter Sunday hours? I'll let the Local try to explain: France So Called Solidarity Day A Complete Sham