what does D-Day mean code example

Example 1: what happened on D-day?

The attack began when Allied planes and warships bombarded German positions 
along the coastline.
At the same time, planes and gliders dropped tens of 
thousands of allied soldiers behind the German defences. They took control of 
important roads and bridges. This made it harder for the German army to rush 
extra men towards areas where the troops were landing.

Example 2: where did D Day happen?

Normandy, France