Ah! It's in the modal that pops up when you add the date, not in the static page. And it's showing up below the bottom of my laptop screen, completely out of sight.
I actually saw it last night, but not this afternoon because my dates were cached and still populating the check-in/out fields. I just clicked submit on that screen without seeing the notice.
So while they're letting us know, the UI could be greatly improved. If I designed this page, I'd show an alert once users add those dates to ensure they're fully aware that the park isn't open.