Quite a bit happened on various April 12ths.
The most important, I think, is that Jonas Salk's polio vaccine was called "safe, effective, and potent" in 1955.
In 1961, Yuri Gagarin became the first human to go into space and the first in orbit.
In 1981, the shuttle Columbia took its first flight.
In 1945, Franklin Roosevelt died, just months before the end of the second World War.
In 1983, Chicago got its first black mayor, Harold Washington.
In 1861, the South continued--and deepened--its treason against the United States by attacking Fort Sumter.