OK, your guess is as good as mine.
I've been searching the interwebs, but I can't find anything that explains what this day means. I've found several sites that mention that it is indeed National Cheese Sacrifice Day, but none that go into detail as to what it is. When Wikipedia doesn't have something, you know it's odd. I guess we are left to speculate.
Does it mean that we buy some cheese and sacrfice it somehow, or does it mean we aren't allowed to eat cheese today, as a sacrifice?
Update: As a reader pointed out in the comments, it's actually Cheese Sacrifice Purchase Day. I've changed the headline but not the URL.