Between the years AD 1900 and 2099, September 11 of the Gregorian calendar is the leap day of the Coptic and Ethiopian calendars. These leap days occur in the years immediately before leap years in the Julian and Gregorian calendars. In all common years of the Coptic and Ethiopian calendars, September 11 is New Year's Day.
Since 2001, the date has been widely known for the terrorist attacks that occurred in the United States and were named after it.
al-Qaeda's suicide airplane attacks on Sept. 11, 2001.