What event was president Bush referring what happened on that day?

President Bush was referring to the September 11, 2001, terrorist attacks, where coordinated attacks by the extremist group al-Qaeda targeted the World Trade Center in New York City and the Pentagon in Washington, D.C. This tragic event resulted in the deaths of nearly 3,000 people and had a profound impact on U.S. domestic and foreign policy, leading to the War on Terror and significant changes in national security measures.