Well, each daily prize is missable. If you don't claim it the day of there's no going back for it, whether you're Premium or not. The daily artwork also only shows up once, but you will be able to see it again once it moves to the Advent Archives. Also, there may or may not be a unique storyline for this year's Advent Calendar that can only be accessed on the days it shows up, but we won't say when (except maybe be super sure to come see the Advent Calendar on these days - 12/5, 12/13, 12/21, 12/28, 12/29, 12/31!