June 6, 1944-the day on which the allies began their invasion of the european mainland during world war ii is also known as:

D-Day, or the Invasion of Normandy.

Explanation:

On June 6, 1944, the Allies stormed the beaches of Normandy to try to stop German advances across Europe. Germany had invaded France and was trying to take more of Europe, so the Allies knew they needed to act fast in order to push back German troops. The Germans knew this attack was coming at some point, but they did not know the location where the Allies were going to attack. The Allies ended up tricking Germany into thinking they were going to invade the northern part of Normandy, but instead they stormed the beaches and pushed their way in.

d. it adversely affected american productivity