July 4, 1776: The United States Declaration of Independence is signed. This document declared the 13 American colonies' independence from Great Britain and laid the foundation for the United States of America.
August 6, 1945: The United States drops an atomic bomb on the Japanese city of Hiroshima. This event marked the beginning of the nuclear age and led to the surrender of Japan and the end of World War II.
November 9, 1989: The Berlin Wall falls. This event marked the end of the Cold War and the reunification of Germany.
September 11, 2001: The terrorist attacks on the World Trade Center and the Pentagon. This event led to the United States' invasion of Afghanistan and Iraq and the War on Terror.
These are just a few examples of days that have had a major impact on the world. Many other days could be listed, and each person may have their own opinion on which day was the most significant.