April 23 is a big beer day in Bavaria and, this year, expect it to be big at Plattduetsche Park Restaurant and Biergarten in Franklin Square, too.

This is the 500th anniversary of one of the world’s oldest laws covering food and drink: the Reinheitsgebot, or Bavarian Purity Law, which mandated that only water, hops and malted barley could be used to make beer.

The reason was simple: a lot of stuff you wouldn’t want in your beer often was added. Yeast and wheat eventually were added to the acceptable ingredients. The law still applies in Bavaria, though not throughout Germany.

Plattduetsche will mark the anniversary on Saturday by pouring a special dunkel beer made by the Long Ireland Beer Company in Riverhead.

Prices will be $7 for one-half liter; $13 for a liter; and $17 for a pitcher of the nutty, malty, mahogany-hued beer.

Dunkel, a dark lager, “was the first beer regulated by the law in 1516,” said Greg Martin, co-owner of Long Ireland. The brewery also has produced hoppy, fruity kolsch beer, a brew from Cologne, for Plattduetsche Park.

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The beer garden menu includes assorted wursts, potato pancakes, burgers, a 10-inch imported Bavarian pretzel, and other casual fare. The restaurant menu highlights German specialties such as sauerbraten, leberkase, loin of pork, smoked pork, braised beef with bacon, and roasted pork shank with mashed potatoes and sauerkraut.

Plattduetsche Park, 1132 Hempstead Tpke., Franklin Square; 516-354-3131, parkrestaurant.com.