May 1: The Opera house of Geneva, Switzerland is almost destroyed in a fire.

May 3: King George VI opens London's Royal Festival Hall as a patron. - The Festival of Britain opens. - The U.S. Senate Committee on Armed Services and U.S. Senate Committee on Foreign Relations begins its closed door hearings into the dismissal of General Douglas MacArthur by U.S. President Harry S Truman.

May 9: Operation Greenhouse: The first thermonuclear weapon is tested on Enewetok Atoll in the Marshall Islands, by the United States.

May 14: The first volunteer-run passenger trains run on Talyllyn Railway, Wales.

May 15: A military coup occurs in Bolivia.

May 21: The Ninth Street Show, otherwise known as the 9th Street Art Exhibition, a gathering of a number of notable artists, marks the stepping-out of the post war New York avant-garde, collectively known as the New York School.

May 23: The Tibetan government signs the Seventeen Point Agreement for the Peaceful Liberation of Tibet with the People's Republic of China.

May 25: The first atomic bomb "boosted" by the inclusion of thermonuclear materials, is tested in the "Item" test on Enewetok Atoll in the Marshall Islands by the U.S. (source: Wikipedia)