Actually, more US Paratroopers died on D-Day than did members of the 4th US Division. The unexperienced C-47 pilots dropped them all over the place - including into forests and flooded plains, where they drowned. The "spread" was 25 square miles, with stray landings up to 10 more miles away from that.
Hah, you're right, of course. A lot of places I posted this question got the answer September 11th, 2001. That ranks as a deadly day...but D-Day, Kasserine Pass, Pearl Harbour, and the first day of the Battle of the Belleau Woods are all significantly more deadly.