A friend recently told me a remarkable story about her grandfather, a WWII vet. It goes like this:
Grandfather, a polish-American man living in Nebraska, never spoke about his service. At least not until very recently. According to her, he had been involved in a failed European invasion, a "practice dday," that had ended catastrophically, with almost everyone killed. One of the few survivors, her grandfather had been forbidden from discussing the operation, or its outcome, to mitigate the impact of such a failure on the war effort/nation. She said the operation had been revealed in recent years via release of some documents overseas, and the U.S. government had recognized her grandfather (as one of, maybe the only? still living survivor) for both his service and many years of silence.
She (admittedly) didn't have a ton more details, but I thought the story was interesting and sounded plausible (re: propaganda/information control). Curious to see if anyone on AskHistorians knows what the real story here could be!
Two incidents come to mind. The first is the landings at Dieppe, but I suspect this is not what he refers to, as only a handful of American commandos were involved. A mostly British-Canadian (with some French and American commandos) force landed at the resort town of Dieppe in France on August 19, 1942, and the operation was mostly a failure, with high losses, and very few objectives achieved before the decision was made to evacuate those still able. As I said though, no Americans, so I don't think this is it.
More likely, it is a reference to Operation Tiger, which was a training maneuver held at 'Slapton Sands' off the English coast on April 28, 1944. The coast in that region was not too different from the region the D-Day landings were to take place so it was being used to practice on. The convoy carrying the troops was not being closely guarded, with only one destroyer close in, and the next nearest escorts some 12 miles away, enough for a flotilla of German E-Boats to slip through and attach in the early hours of the morning, sinking three LSTs. Some 700 sailors and soldiers were confirmed dead, including several officers who knew the entire D-Day plan, which caused serious worries until all of their bodies were recovered from the wreck.
That wasn't the only disaster during "Tiger" though. During the practice landings, when deposited on the beach the troops were supposed to be preceded by artillery fire, but due to organizational errors, the artillery that was supposed to stop before they landed continued, and added more to the casualty rolls, bringing the total killed from the Slapton Sands debacle to 946, considerably higher than were lost on many of the actual D-Day beaches! It did have some "positive" effects though, as the Army learned some lessons for the real deal.
As you note, the whole thing was kept hush-hush, with court-martial facing anyone who talked, and information was not released for several decades, although it doesn't seem to have been strictly speaking, classified.
Rick Atkinson, Guns at Last Light pages 16-17
One possible match would be the raid at Dieppe. It was undertaken by the Allies and featured a large Canadian contingent. In a way it was a "practice D-Day," as it did try out some invasion techniques. It achieved few of its objectives, but as a raid it was never intended as an invasion. It was also costly, but I don't know of any secrecy around the operation.
An operation that was hushed up was Excercise Tiger, which was a practice D-Day. The simulated assault was in the UK (Slapton Sands), not France. It had a tragic amount of friendly fire as well as attacks by German E-boats. The resulting cost was nearly a thousand dead. There is some debate as to whether it was covered up or merely forgotten in the grander scheme of the war, but it certainly did not feature prominently in heroic memory.
If he served in the US Army he is likely remembering "Exercise Tiger" it was a training operation in April 1944, designed to practice and figure out best practices for landing lots of crafts from different services, nations, and sizes, and timing things like shore bombardment. Unfortunately it just wasn't practical to bring ships from the Pacific, and it would have been unnecessarily complex. the plan was to land several thousand US troops from British ships in Devon in SW England.
The day as a disaster. A RN Cruiser, the Hawkins due to a delay in H-Hour, was firing live shells onto the beach when US troops had already landed(the landing craft hadn't heard about the delay). Then the worst of it was when German E-Boats attacked a convoy of LST's bringing the second wave of troops for the exercise. All told about 1,000 servicemen were killed.
The deaths were not released immediately (which was extremely common in lots of circumstances such as an important ship sinking) and released during the summer as part of the casualties from Overlord itself.