'Twas the night before D-Day. One ship, carrying Special Force Six, leaves ahead of the main invasion on a dangerous mission. On board are British Colonel Wynter and American Captain Parker, who each, in flashback, reminisce about their separate involvements with beauteous Valerie Russell. Will the coming battle (confined to the film's last fifteen minutes) determine which one comes home to her?
Synopsis by IMDb
D-Day the Sixth of June is an American romantic film drama first released in 1956, directed by Henry Koster. The film is based on a novel by Lionel Shapiro and stars Robert Taylor, Richard Todd, Dana Wynter and Edmond O'Brien and John Williams. It is a Twentieth Century Fox Film Corporation production, produced by Charles Brackett.
Our overall rating for D-Day the Sixth of June is: good.

Although it was released six years before The Longest Day, D-Day The Sixth Of June has had to live in the shadow of Darryl F Zanuck's star-studded epic. That Henry Koster's picture has been overlooked has a lot to do with the scale of Zanuck's production and that cast - in The Longest Day, even the lowest ranked Tommies are played by Sean Connery. Since Koster is something of a master of the form (he made Harvey, lest we forget), it's a shame his picture's gone AWOL. That it stars real-life legend Richard Todd makes the lack of attention sadder still.
As has been pointed up elsewhere,Richard Todd was cut from particularly resilient cloth. A paratrooper throughout WWII, Todd was commended for his role in capturing the Orne Bridge during the D-Day assault. In The Longest Day, he actually plays the other officer involved in that key maneuver. In The Sixth Of June he plays Lieutenant Colonel John Wynter who, together with Robert Taylor Captain Parker, takes time to remember the joys and hardships of his life as they approach the Normandy coast.
Since The Sixth Of June is an American movie, Todd pretty much plays second fiddle to Robert Taylor, arguably the dullest of all Hollywood leading men. And as Koster has little budget to work with, there's little in the way of Saving Private Ryan-style spectacle - the film is more concerned with which man will return to Dana Wynter than with accurately portraying the landings. It's Todd, though, who lends the picture the gravity that prevents its descent into melodrama. And likewise it's director Koster who winds up achieving quite a lot despite having relatively little to hand.

Richard Todd,real life hero of D-Day.
The Longest Day definitely has the edge, but there are lots of good reasons to make a date with The Sixth Of June.

Along with the account of the significant historical event, the picture spins an extremely moving wartime love story, distinctively done by a finely performing cast. While the atmosphere and threat of war are always present in the topnotch Charles Brackett production, it isn’t until near the end that actual fighting is shown in all of its frightening detail.