One of the things about holidays and work in France: if a holiday falls on a day you don't work anyway, you still get to take the holiday, at some later date, at your leisure!
I mean, really. It's not fair for a day off to fall on a day off. You can't relax twice as much that day. It's not fair to look at your neighbor and see he's having a delicious day off when he should have been slaving away, and here you are having, sure, a day off, but you would have had the day in any case. It's just not the same.
So the law says your employer has to give you a working day off later.
14 years and this never ceases to amaze me.