I agree with Dangerosa. “Casual” is a miserably unclear way to attempt to communicate “no jeans, dammit,” and is in fact probably the very best way to guarantee that there will be people in jeans. It would be a 50/50 chance whether I’d be one of those people, depending on what happened to be on the very top of the laundry stack. I wouldn’t specifically root through the dresser looking for cords when there jeans right to hand–it doesn’t get less dressy than casual, so if I’m fit to go shopping, I’m fit to go to your party. “Holiday casual” means I would find a blinking nose to wear with the antler headband I wear running errands anyway.
Otoh, if you said “office casual” I would root, because that is clearly and unambiguously understood to mean “no jeans, dammit.”
I don’t understand carpet bans against “jeans” anyway. A nice* pair of well-fitting dark jeans paired with a luxe top, and/or even better, a great jacket, is infinitely “dressier” than khakis and a polo or sweater.
*Of course, I do understand that “nice” is a relative term, but it applies equally relatively across the board, and I see just as many people looking sloppy in ill-fitting, ratty khakis or black pants as I do in ill-fitting ratty jeans.
After seeing Jim’s staff Christmas party this weekend and seeing people in everything from super-formal dresses (full-length, sparkly, flouncy, etc.) to a white cotton top and black cotton pants (on a woman), I’m coming to realize that it doesn’t really matter what people wear - if you care about being appropriate, aim for appropriate, and if you don’t care, wear whatever you want.