DELIVERANCE? Destiny? Decision? Determination? Dooms-? The question I've been asked most this week is what does the D in D-Day stand for?

A glaring omission from our coverage, we haven't answered it. That's because, according to my research at least, it doesn't stand for anything.

The planning for the invasion of France began as early as 1941, and really got going in mid-1942. None of the planners knew when precisely the invasion would happen, but they had to work out a sequence of events: if you land on day one, how many men and what differing sorts of equipment are you going to need in the subsequent days if the invasion is going to be sustainable.

So, rather like one of those interminable quadratic equations from schooldays, if the day of the landing is D, what happens on D+1, D+2...?

The date of D-Day was a moveable feast, right up to the moment it happened. In late May 1944, everyone assumed it would be, as planned, on June 5 when the new moon meant tides would be at their highest.

But early June was wet and windy, with depression after depression scudding across the Atlantic to batter northern Europe.

The leaders, General Dwight Eisenhower and General Sir Bernard Montgomery, were conjuring with the dangers of postponing the operation altogether. The next suitable tides weren't for another two weeks, but what would do you do with the thousands of men already bobbing in boats off Southampton? And wouldn't the delay allow the Germans to get wind of their plans? As it turned out, the enemy already thought an operation called Overlock was about to begin (very close to the "Overlord" codename that the Allies were using).

On the evening of June 4, Group Captain James Stagg reported to Eisenhower and Montgomery. He was the weatherman, a junior officer. He was the man who had to juggle the elements and the differing requirements of the services: paratroopers couldn't land in more than 20mph winds; naval bombers wanted winds less than 12mph onshore and 18mph offshore; the transport aircraft wanted clouds no lower than 2,500ft.

But he, in the end, was the man who set D-Day as June 6.

He told the leaders that, according to his chart, an unseasonal cold front had accelerated as it crossed the Atlantic and was due to hit on June 5. But the wet front behind it was beginning to slow. The gap between the two was increasing from 24 hours towards 48.

Eisenhower, with Montgomery's agreement, decided that the day, D-Day, would be postponed for 24 hours and slotted into this weather window.

Thoughts then turned to H-Hour: the time on D-Day on which the operation would begin.

The Germans were expecting an invasion at around high tide when the beach was at its narrowest. Therefore, they had built all sorts of defences to destroy boats and blow up men around the high-water mark.

So the Allies decided not to set H-Hour for high tide. And they didn't want it at low tide because it would have meant landing craft getting stuck on sandbanks. It would also have given the men vast stretches of beaches to run up and get mown down on.

So they set H-Hour on a rising tide which would set the men down about 300 metres from the Germans' high-water defences. Because the landings were on 50 miles of beaches, each beach had a different H-Hour to suit the tide.

Although time and tide wait for no man, the Allies used both to their fullest advantage on D-Day.