April 17 1961:

The Bay of Pigs invasion begins when a CIA-financed and -trained group of Cuban refugees lands in Cuba
and attempts to topple the communist government of Fidel Castro.
The attack was an utter failure.

April 20 1914:

Ending a bitter coal-miners' strike, Colorado militiamen attack a tent colony of strikers,
killing dozens of men, women, and children.

Although the "Ludlow Massacre" outraged many Americans,
the tragedy did little to help the beleaguered Colorado miners and their families.
Additional federal troops crushed the coal-miners' strike,
and the miners failed to achieve recognition of their union or any significant improvement in their wages and working conditions.
Sixty-six men, women, and children died during the strike, but not a single militiaman or private detective was charged with any crime.

April 29 2004:

The National World War II Memorial opens in Washington, D.C., to thousands of visitors,
providing overdue recognition for the 16 million U.S. men and women who served in the war.
The memorial is located on 7.4 acres on the former site of the Rainbow Pool at the National Mall between the Washington Monument
and the Lincoln Memorial.
The Capitol dome is seen to the east, and Arlington Cemetery is just across the Potomac River to the west.

April 30 1945:

Holed up in a bunker under his headquarters in Berlin,
Adolf Hitler commits suicide by swallowing a cyanide capsule and shooting himself in the head.
Soon after, Germany unconditionally surrendered to the Allied forces,
ending Hitler's dreams of a "1,000-year" Reich.

On May 4:

1970 – Vietnam War: Kent State shootings: the Ohio National Guard, sent to Kent State University after disturbances in the city of Kent the weekend before, open fire killing four students and wounding nine others. The students were protesting the United States' invasion of Cambodia.