On 27 May 1937, the Golden Gate Bridge opened to pedestrians. Cars would wait until the 28th!
On 25 May 1979, six-year old Etan Patz never made it to school. His became the face of missing children in the 1980s.
DST was a WWI effort. Germany was the first, in April 1916. The UK followed on 21 May 1916.