The D-Day date set

On 8 May 1944 General Dwight Eisenhower, Supreme Commander of the Allied Expeditionary Force, fixed the day of the Normandy landings. The invasion of France, Operation Overlord, was to begin on 5 June. The American official history (Gordon A. Harrison, Cross-Channel Attack) records the decision plainly: on that day Eisenhower set D-Day as “Y plus 4” – four days after the so-called Y-Day, the date by which all forces were to be ready, which fell on 1 June. The date of 8 May is therefore correct. Some accounts, however, link the final, “irrevocable” fixing of 5 June with 17 May; the difference may arise because the date set provisionally on 8 May was later merely confirmed.

The date was not an arbitrary choice. It was the result of a calculation in which the phase of the moon, the time of sunrise and the height of the water on the French shore all counted. Only a few days a month met all the requirements at once, and 5 June was the first of them.

From May to June

Originally the invasion was to take place in the spring. At the Quebec Conference in August 1943 the Allied chiefs of staff adopted a target date of 1 May 1944, and in November, at the Tehran Conference, Franklin Delano Roosevelt and Winston Churchill assured Joseph Stalin that the landing would come in May. In return Stalin promised a simultaneous offensive in the east, which was to prevent the Germans from transferring divisions to the west.

The plans changed when, in January 1944, Eisenhower and General Bernard Law Montgomery widened the assault from three to five divisions in the first wave. This required far more landing craft and landing ships, which were in short supply in every theatre of the war. Eisenhower, who had only just arrived in London, asked for a month’s delay: an extra month of shipyard work meant more landing ships for the first wave. At the end of January the chiefs of staff agreed to put the invasion back to the beginning of June. The spring was not wasted, though – it benefited the air forces, which in April and May were destroying French and Belgian railway junctions and, from the beginning of May, were stepping up their bombing of targets ahead of the invasion.

Around the turn of April and May the preparations for the invasion were entering their final phase, and the troops were completing their exercises. After April’s Exercise Tiger, which ended in the tragedy at Slapton Sands, a series of Fabius exercises was held at the beginning of May – a dress rehearsal for the British, Canadian and American assault forces. The plan was ready, the troops were trained, and SHAEF headquarters could at last name the day.

Moon, tide and dawn

All three services had their requirements. The navy under Admiral Bertram Ramsay wanted to approach the shore in darkness but to bombard the defences and land the infantry in daylight – so H-Hour had to fall shortly after dawn, to let the naval gunners and airmen see their targets. The air forces commanded by Air Chief Marshal Trafford Leigh-Mallory needed a moon on the night before the landing: three airborne divisions – two American and one British – were to be dropped behind the beaches, and the pilots of the transport aircraft and gliders had to be able to see the ground. A night close to full moon was best.

The third condition – the water – was the hardest. Ever since Field Marshal Erwin Rommel had taken charge of the Atlantic Wall, the Germans had been studding the beaches with thousands of obstacles: wooden stakes, steel “hedgehogs” and trestles, often with mines attached. At high water the landing craft would run onto them unseen beneath the surface. The planners therefore decided to land on a low but rising tide: the obstacles would then stand on dry sand where engineers could blow them up, and the incoming tide would lift the craft closer to the shore by the hour and let them pull away after unloading. The price was a long run for the infantry across the open beach. On top of the tides in the Bay of the Seine came a difference in timing: the water rose earlier in the west than in the east, so the Americans were to land at 6.30 a.m. and the British and Canadians about an hour later.

When all these conditions were set against the calendar, it turned out that in the first half of June only 5, 6 and 7 June met them. The next chance – with suitable tides but no moon – came only on 18–20 June. Eisenhower chose the first day of the window. If the weather failed, two more remained in reserve.

Y-Day and D-Day

In staff language “D-Day” simply meant the day the operation began, and “H-Hour” the time of the first landing; all the timings in the plan were counted from them (“D plus 1”, “H minus 30”). Y-Day was an auxiliary concept introduced for the purposes of logistics: by 1 June all the units of the first waves were to be standing ready in camps near the ports. The decision of 8 May gave the whole machine its final rhythm. From then on the quartermasters knew when the columns were to set off from the concentration areas for the coast, and the naval commands knew when to begin loading.

The date was the most closely guarded secret of the war. It was known only to a small circle of officers with “Bigot” clearance, and most soldiers learned the objective of the operation only in the last week of May, when they were shut inside guarded camps. According to the American official history, the American assault forces began boarding ship on 30 May, and by 3 June everyone was aboard.

Weather – the unknown in the equation

The moon and the tides could be calculated years ahead; the weather over the English Channel could not. The decision of 8 May was therefore conditional: the final order was to be given only at the last moment, on the strength of a forecast. Forecasting was the responsibility of SHAEF’s chief meteorologist, the Scot James Stagg – a civilian scientist from the British Meteorological Office who in the autumn of 1943 had been given the rank of group captain in the RAF Volunteer Reserve so that the military would listen to him. Stagg did not forecast alone: he collated the assessments of three teams – the Meteorological Office, the Admiralty and the US Army Air Forces – which often disagreed, and presented a common position to the commanders. His deputy was the American Colonel Donald Yates.

According to Stagg’s account, in May Eisenhower regularly listened to trial forecasts to see how accurately the meteorologists could predict the weather several days ahead. According to the official history, on 29 May Stagg presented an optimistic long-range forecast for the first week of June, and the preparations were completed on that basis. The final conferences were held at Southwick House near Portsmouth, Ramsay’s headquarters, close to the embarkation ports.

What the Germans expected

The German high command knew that the invasion was near, but knew neither the place nor the day. The Commander-in-Chief West, Field Marshal Gerd von Rundstedt, and most of the generals expected the main blow across the Strait of Dover, where the sea crossing was shortest; they were confirmed in this by the Allied Operation Fortitude, with its phantom army in south-east England. Adolf Hitler had already pointed in March to the Cotentin peninsula and Brittany as possible targets, and at the end of April he began pressing for Normandy to be reinforced. On 6 May Rommel passed on his concern to the Seventh Army, and over the following days the 91st Air Landing Division (91. Luftlande-Division) was sent to the Cotentin and the 6th Parachute Regiment to the area of Cherbourg. Rommel, who commanded Army Group B, toured Normandy in May and speeded up the work there, although – as most historians accept – like Rundstedt he considered the coast of the Pas-de-Calais department the most likely target of the main blow.

The Germans inferred the timing mainly from the weather and from the air raids. The naval commander in the West, Admiral Theodor Krancke, believed that the invasion would be preceded by much heavier bombing of the coastal batteries; as late as 31 May he judged that the attacks were too weak for that, and although he himself pointed to the threat to the Cotentin, he still expected a great landing in the Pas-de-Calais. German meteorologists, deprived of stations in the Atlantic, where the Channel weather came from, could not foresee the brief improvement that Stagg detected.

Place in the war

The day fixed on 8 May was not kept. On the evening of 3 June Stagg forecast a storm, strong winds and low cloud for 5 June. Early on the morning of 4 June Eisenhower postponed the invasion by twenty-four hours – some of the convoys were already at sea and had to turn back. That evening, at a conference in the library of Southwick House, Stagg predicted a brief clearance. The next window would not come until 19 June, with no moon for the paratroopers, and the troops would have had to be disembarked, risking the loss of secrecy. Eisenhower gave the order, and in the early hours of 5 June, after another forecast, confirmed it. On 6 June 1944 the Allies landed in Normandy.

Surprise was in large part the work of the weather: the German navy called off its patrols for the night of 5/6 June, and Rommel left for home in Germany, from where he intended to go on to see Hitler. The reserve date, moreover, would have proved disastrous – on 19–22 June a storm raged over the Channel that wrecked the American artificial harbour off Omaha. The date set on 8 May also tied East to West: some two weeks after the landing, on 22 June, the Red Army opened its great summer offensive in Belarus, fulfilling the Tehran promise of a blow from both sides.