DELIVERANCE? Debarkation? Destiny? Decision? Determination? Dooms-? What does the D in D-Day stand for?

Well, nothing really. Unlike most of the other codenames deployed 75 years ago, the principal one by which we know the operation doesn’t mean anything more than “day”.

The whole project was known as Operation Overlord, a grand Churchillian name for the largest seaborne invasion of all time. The naval part of the operation was codenamed Neptune, after the Roman god of the sea.

The US build-up of troops in Britain was known as Operation Bolero because, like the Latin music, it was going to have a slow but deliberate pace and grow to a crashing climax.

And there was a big propaganda operation designed to convince the Germans, through the creation of phantom armies complete with inflatable tanks in Kent, that the invasion was going to come via Calais with a secondary attack on Norway. The propaganda blitz was known as Operation Bodyguard after Churchill had told Joseph Stalin: “In wartime, truth is so precious that she should always be attended by a bodyguard of lies.”

One of the few codenames that was meaningless was “mulberry”, the artificial harbours that would be crucial for landing heavy equipment. At first, these were referred to in documents as the “artificial harbour” but when the security risk was realised, a list of random words was consulted. Top of the list was “mulberry”, and so the harbours were codenamed. Other words on the list were used to generate codenames for parts of the harbours: the blockships on which they were to be anchored were called “corncobs”, the outer breakwaters were “gooseberries”, the caissons were “phoenixes”, the roadways were known as “whales” and “beetles”, and the pierheads were “spuds”.

No one knew precisely when the mulberries would be required, but they began to work out what supplies would need to be delivered over them in the days after the invasion to ensure the soldiers could fight ahead. The first day after the landing was known as D+1, the second was D+2...

So the day itself became D-Day – but no one knew when it was going to be.

For D-Day to be successful, it needed a full moon to shed light on proceedings and to ensure a high tide. June 5 was pencilled in, but as it approached, the weather worsened, with depression after depression scudding across the Atlantic to batter northern Europe. D-Day was going to be postponed for a fortnight until the next suitable D-Day.

But on the evening of June 4, weatherman Group Captain James Stagg reported a window. An unseasonal cold front had accelerated as it crossed the Atlantic and was due to hit on June 5 – the weather was so bad that the German commander in Normandy, Erwin Rommel, thought no invasion was possible and returned to Germany with a pair of shoes as a 50th birthday present for his wife.

But Stagg had noticed the wet front behind the cold front was beginning to slow creating a gap that was increasing from 24 hours towards 48. So a weatherman set the date for D-Day by postponing D-Day by a day to June 6, 1944. Operation Neptune began, the mulberries were towed into place, and Operation Bodyguard had throw the Germans off the scent of Operation Overlord – although they knew something was coming, but thought it would be called Operation Overlock.