...or Tag Des Deutschen Bieres as it's called in Germany, is today--April 23rd.
Way back in 1516, Frederik I of Bavaria declared you can only use four ingredients to brew beer: water, malt, hops, and yeast. It's known as the Bavarian (or German) Purity Law, or Reinheitsgebot. Although in 1993, the law was modified a bit so if you decide you want to use wheat or a couple other ingredients in your beer, they won't haul you off and execute you or anything.
So the next time you lift that stein and belt out "Ein Prosit", send up a thanks to old Freddy of Bavaria.