There's a ridiculously long list of what's happened on the 20th of May in history, so let's go with two major facts and a small history post for each. I think it'd be more interesting than a list of briefly mentioned things that happened on this day.
1961 -- astronaut Alan B. Shepard Jr. became America's first space traveler. He made a 15-minute suborbital ballistic flight in a capsule launched from Cape Canaveral, Florida. The space craft was called Freedom 7, launched by a Redstone vehicle. He went 116 statute miles into the air and landed 302 statute miles down the Atlantic Missile Range.
1985 -- President Ronald Reagan attended a wreath-laying ceremony at a military cemetery in Bitburg, West Germany. The visit drew worldwide condemnation because 49 members of the Waffen SS were buried there. For those of you who don't know, the Waffen SS was the armed wing of the Schutzstaffel, or Protective Squadron. They were one of the largest sections of the Nazi forces.
Another little fact that's interesting, is that Karl Marx was born in Prussia in 1818, and Napoleon Bonaparte died in exile on the island of St. Helena in 1821.