Wikipedia:Selected anniversaries/April 20
- 1862 - Louis Pasteur and Claude Bernard completed the first test on pasteurization .
- 1912 - Fenway Park in Boston, Massachusetts and Tiger Stadium in Detroit, Michigan were opened.
- 1914 - A fire and a gun battle between the National Guard and striking coal miners in Colorado led to 17 deaths in the Ludlow Massacre.
- 1968 - Pierre Trudeau succeeded Lester B. Pearson as Prime Minister of Canada.
- 1999 - Eric Harris and Dylan Klebold murdered 13 people in the Columbine High School massacre.