What was the single highest day of American casualties in World War 2?

For single-day totals, the biggest was the first day of Operation Overlord (D-Day), June 6, 1944. Modern estimates are 2500+ American Kia, with 3x Wounded. This is significantly higher than previous records estimated.

While many of the Pacific campaign battles were vicious, they ultimately produced few casualties, as the number of US troops involved was severely limited. The Battle of Tawara (Nov 20-23, 1943) probably has the largest number (1300 KIA, 2300 WIA) over the shortest period. The Battle of Okinawa (April 1 - June 22, 1945) produced a very high casualty rate for a single "battle" (~50,000 total), but this is similar to the Normandy Campaign (June 6 - August 31, 1944), which produced 100,000+ American casualties.