I know this will piss off most of the folks here, but DDay was probably not the greatest day in the War.
You could look at Iwo, or Okinawa. You could make a case for the battle of Stalingrad, or the tank battles in Ukraine. DDay would not have been possible without the Battle of Britain, or the Battle in the Atlantic.
It is important to remember that it was a world war, and that the invasion of Europe created a “third” front after the east and south.
Lots of places, and lots of people did incredible things to banish fascism from Europe.
It is important to honor them all.
No other nation could have amassed that armada. It was an amazing achievement. So yes it was the greatest day of the war but not its biggest battle. Not by a long shot.