It certainly is - but in both cases - it just goes to show how unwieldy it would get to calculate every single holiday based on some crazy set of rules. If you're going to go through all of that trouble - calculate it and store it (never mind the bizarre add-on rules, like "observe the holiday on the monday if it falls on a sunday, except if it's a minor holiday which might be defined by theses x rules in the HR handbook...")

And Joe's point is a good one. there are multiple definitions of a lot of these....But even just using the definitions your org has picked out, seems silly to have to figure them out over and over again.