How was D-Day a big turning point for World War 2?

Operation Neptune-Overlord/D-Day represented the opening of the long-awaited "Second Front." With the Soviet Union and supporting partisan forces engaged in heavy, costly fighting on the Eastern Front, the Western Allies' invasion forced Nazi Germany to siphon off valuable resources to meeting this new threat.

Secondly, it was a symbolic gesture, a stunning reversal of the Allies' pell-mell defeat and narrow escape at Dunkirk in 1940; reminding Nazi Germany once again that the Allies would not back down from the conflict at hand.