D-Day Invasion of Europe by the Allies. What does the "D" stand for in D-Day?

It does actually stand for anything.
D-Day - Wikipedia, the free encyclopedia

I no it didn't stand for dooms day, or death day like everyone thinks it does. But doesn't it stand for Designated Day?

It doesn't stand for anything. There are actually several "D-Days" in history. The official name for the D-Day invasion was Operation Overlord, not D-Day. D-day simply means the date that something is to happen. It's a title used in anticipation of the day.