April 12th has been a very big day in history, and I'm hard pressed to...

...think of a bigger one over the years:

1633 Galileo is convicted of heresy

1861 U.S. Civil War Begins

1954 Bill Haley and the Comets record "(We're Gonna) Rock Around the Clock"

1961 Yuri Gagarin becomes the first man in space

Other suggestions?