This is some other stuff about Germany that is good to know, but which I forgot to add in earlier into the customs thing.
Most Germans will understand if you pronounce their currency unit like it looks you should. But unless they check themselves, they may well say it back to you sounding like "oi row", as per the punk subgenre and the thing you do with a boat. Since you're going to be doing a lot of euro-spending for transit, merch, and provisions, this is something you should probably be prepared to hear.
In Germany, the concept of 'disposable' is highly deprecated. All bottled and canned beer, all canned soft drinks, and almost all bottled soft drinks have some kind of return deposit on the containers, which is not included in the billed price. This return deposit, called Pfand, which you'd probably never associate as such, is also extended to drinks cups at gigs and festivals, at which Wacken is no exception. Thus your first beer will cost 4e, even though the beer is only 3e, because you also have to pay for the cup. If you trade the empty cup in for a full one, you're effectively paying 'only' for the beer inside, and it will be 3 euro. When you get a litermug of something, the deposit is 3.50e.
The upside of this, of course, is that you're allowed to keep the cups that you technically buy, and they're solid enough to make this worthwhile. Every year the logo design on the liter mug and hefeweizen glasses (from the beergarden) is different, and every year there are about 10 different designs used for the 0.4- and 0.3-liter cups used for beer and mixed drinks (respectively). If you are easily addicted to collectible-commodity games like Magic, Pokemon, or limited-edition vinyl, follow these with caution, because the effort of collecting the whole set will significantly obstruct your ability to actually see bands like you came to this festival to do.