As opposed to international beer day, which is celebrated in August, Germany’s beer day is set on this date to commemorate the passage of the Reinheitsgebot (or the Purity Law) on April 23rd, 1516. The Purity Law stated that beer may only consist of water, malt, hops and yeast.

The original law didn't explicitly allow yeast as ingredient, as it was still "unknown".

is this really the most important thing to have here, beed is already bought from eu