I still can't tell if you're being sarcastic, I truly can't. I'm just going to assume that you are, because it's better for the entire planet that way.
You really need it explained to you? Okay, here goes. The two scenarios are:
1) There is a day, planned in advance, where the teachers know that the entire class will not be present. The teachers can plan around this to ensure that the the right work is carried out for the class to catch up...