I've always been tremendously fond of Mount Holyoke College's Mountain Day. By tradition and necessity, Mountain Day has no fixed date, but is declared on the spot by whatever authority is charged with the decision. Whenever it falls, it is a day of brilliant autumn weather, too beautiful to be spent in ordinary indoor pursuits. On Mountain Day all classes and labs are cancelled, any deadlines rolled over to the next day, and everyone is released to go climb a pint-sized New England mountain or pick apples or whatever. (I think, but am not sure, that the general holiday extends to virtually all staff as well.)

I'd totally put Mountain Day on a national holiday list. For that matter, I'd add Ice Day for a perfect winter day, Flower Day for a perfect day in Spring, and perhaps Meteor Night for summer.