On April 20:
- 1657 - New Amsterdam (later to become New York) granted freedom of religion to Jews.
- 1862 - The first pasteurization test was completed by Louis Pasteur and Claude Bernard.
- 1902 - Pierre and Marie Curie refined radium chloride.
- 1914 - The Ludlow Massacre of striking Colorado coal miners and their families took place.
- 1999 - Eric Harris and Dylan Klebold murdered 13 people in the Columbine High School massacre.
On April 20:
- 1657 - New Amsterdam (later to become New York) granted freedom of religion to Jews.
- 1862 - The first pasteurization test was completed by Louis Pasteur and Claude Bernard.
- 1902 - Pierre and Marie Curie refined radium chloride.
- 1914 - The Ludlow Massacre of striking Colorado coal miners and their families took place.
- 1999 - Eric Harris and Dylan Klebold murdered 13 people in the Columbine High School massacre.