Don't understand

One thing I dont understand about schools lately is that they actually have school days that say "Non-student day". So kids dont have school for that day!! How can you have a non-student day? Isnt school about students?!! I find this very irritating. I am sure they have a reason for these days, but I honestly can't say I ever had a "non-student" day. Of course now I sound like an old person with the "back in my day..." It is only the matter of time before I start those stories with my kids. I think it is in the parent handbook or something.