I saw a blurp on the news about this to include Hitler's birthday (April 20, 1889).
Many tragedies during april:
April 14, 1865 - President Abraham Lincoln assassinated at Ford's Theatre
April 20, 1889 - Adolf Hitler was born.
April 4, 1968 - Martin Luther King Jr., assassinated
April 19, 1993 - The 50 day siege at Waco, TX
April 19, 1995 - The Oklahoma City bombings
April 24, 1995 - The "Unabomber," (Ted Kaczynski), his last victim was killed April 24, 1995.
April 20, 1999 - Columbine High School shooting (killed 12 students and one teacher, 21 injured)
April 16, 2007 - Virginia Tech shooting (killed 32 people and hurt 17 others)
April 15, 2013 - Boston Marathon bombing (killed 3 injured about 170)