historical and cultural costumes only......
My kids last school had that policy, they had a cultural day instead. And had an outdoor track day on halloween, it's usually snowing by then.. good idea. I think it is silly personally, and it is part of our culture.
While there is nothing wrong with cultural day, I do feel bad for the kids that have no traditional cultural clothing, a lot of us are very mixed breeds far removed from any tradition for generations. I don't know if I am supposed to send my kids in Canadian clothes on those days, jeans.. t-shirt and hoodie.