On April 23, 1516, in Ingolstadt, William IV of Bavaria issued the “Reinheitsgebot.” It is a major milestone in the legislation that regulates the production of beer. That edict sanctified the ingredients with which beer should be brewed: “Gersten, Hopffen und Wasser.” Yeast was not mentioned because it was not yet identified by science as a specific product. Of course, it was already used by the brewers, but why would they tell the King? This law was implemented in Bavaria. The rest of actual Germany was not submitted to that law

So that is why April 23 is every year a big “beer-fest” day in Germany.
Newsletter April, 2001