On 17 September 1787, 42 of the 55 delegates to the Philadelphia convention held their final meeting, to sign the document which the convention had produced, the United States Constitution.
On 17 September 1862, Union forces under General George McClellan and Confederate troops under General Robert E. Lee fought the Battle of Antietam near Sharpsburg, Maryland. (The south would call it the Battle of Sharpsburg.) This date is still the bloodiest single day in American military history. Although the Battle of Gettysburg took more lives, it was spread out over three days, none of which equalled the single-day total at Antietam.