The invasion planners determined a set of conditions involving the phase of the moon, the tides, and the time of day that would be satisfactory on only a few days in each month.
A full moon was desirable, as it would provide illumination for aircraft pilots and have the highest tides.
The Allies wanted to schedule the landings for shortly before dawn, midway between low and high tide, with the tide coming in.
This would improve the visibility of obstacles on the beach, while minimising the amount of time the men would be exposed in the open.
Eisenhower had tentatively selected 5 June as the date for the assault.
However, on 4 June, conditions were unsuitable for a landing: high winds and heavy seas made it impossible to launch landing craft, and low clouds would prevent aircraft from finding their targets.

Group Captain James Stagg of the Royal Air Force (RAF) met Eisenhower on the evening of 4 June.
He and his meteorological team predicted that the weather would improve enough for the invasion to proceed on 6 June.
The next available dates with the required tidal conditions (but without the desirable full moon) would be two weeks later, from 18 to 20 June.
Postponement of the invasion would have required recalling men and ships already in position to cross the Channel, and would have increased the chance that the invasion plans would be detected.
After much discussion with the other senior commanders, Eisenhower decided that the invasion should go ahead on the 6th.
A major storm battered the Normandy coast from 19 to 22 June, which would have made the beach landings impossible.

Allied control of the Atlantic meant German meteorologists had less information than the Allies on incoming weather patterns.
As the Luftwaffe meteorological centre in Paris was predicting two weeks of stormy weather, many Wehrmacht commanders left their posts to attend war games in Rennes, and men in many units were given leave.
Field Marshal Erwin Rommel returned to Germany for his wife's birthday and to meet with Hitler to try to obtain more Panzers.
The invasion planners determined a set of conditions involving the phase of the moon, the tides, and the time of day that would be satisfactory on only a few days in each month. A full moon was desirable, as it would provide illumination for aircraft pilots and have the highest tides. The Allies wanted to schedule the landings for shortly before dawn, midway between low and high tide, with the tide coming in. This would improve the visibility of obstacles on the beach, while minimising the amount of time the men would be exposed in the open. Eisenhower had tentatively selected 5 June as the date for the assault. However, on 4 June, conditions were unsuitable for a landing: high winds and heavy seas made it impossible to launch landing craft, and low clouds would prevent aircraft from finding their targets.
Group Captain James Stagg of the Royal Air Force (RAF) met Eisenhower on the evening of 4 June. He and his meteorological team predicted that the weather would improve enough for the invasion to proceed on 6 June. The next available dates with the required tidal conditions (but without the desirable full moon) would be two weeks later, from 18 to 20 June. Postponement of the invasion would have required recalling men and ships already in position to cross the Channel, and would have increased the chance that the invasion plans would be detected. After much discussion with the other senior commanders, Eisenhower decided that the invasion should go ahead on the 6th. A major storm battered the Normandy coast from 19 to 22 June, which would have made the beach landings impossible.
Allied control of the Atlantic meant German meteorologists had less information than the Allies on incoming weather patterns. As the Luftwaffe meteorological centre in Paris was predicting two weeks of stormy weather, many Wehrmacht commanders left their posts to attend war games in Rennes, and men in many units were given leave. Field Marshal Erwin Rommel returned to Germany for his wife's birthday and to meet with Hitler to try to obtain more Panzers.
Normandy landings